Find all School-related info fast with the new School-Specific MBA Forum

It is currently 29 Aug 2014, 16:21

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

Math: Number Theory (broken into smaller topics)

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
Expert Post
5 KUDOS received
Math Expert
User avatar
Joined: 02 Sep 2009
Posts: 25206
Followers: 3418

Kudos [?]: 25033 [5] , given: 2702

Math: Number Theory (broken into smaller topics) [#permalink] New post 10 Mar 2010, 05:20
5
This post received
KUDOS
Expert's post
1
This post was
BOOKMARKED
NUMBER THEORY

This post is a part of [GMAT MATH BOOK]

created by: Bunuel
edited by: bb, walker

--------------------------------------------------------
Image

The information will be included in future versions of GMAT ToolKit
[read more] [AppStore]

--------------------------------------------------------

DEFINITION

Number Theory is concerned with the properties of numbers in general, and in particular integers.
As this is a huge issue we decided to divide it into smaller topics. Below is the list of Number Theory topics.

GMAT Number Types

GMAT is dealing only with Real Numbers: Integers, Fractions and Irrational Numbers.


INTEGERS

Integers are defined as: all negative natural numbers \{...,-4, -3, -2, -1\}, zero \{0\}, and positive natural numbers \{1, 2, 3, 4, ...\}.

Note that integers do not include decimals or fractions - just whole numbers.

Even and Odd Numbers
Prime Numbers
Factors
Finding the Number of Factors of an Integer
Finding the Sum of the Factors of an Integer
Greatest Common Factor (Divisior) - GCF (GCD)
Lowest Common Multiple - LCM
Divisibility Rules
Factorials
Consecutive Integers
Evenly Spaced Set


IRRATIONAL NUMBERS

Fractions (also known as rational numbers) can be written as terminating (ending) or repeating decimals (such as 0.5, 0.76, or 0.333333....). On the other hand, all those numbers that can be written as non-terminating, non-repeating decimals are non-rational, so they are called the "irrationals". Examples would be \sqrt{2} ("the square root of two") or the number pi (\pi=~3.14159..., from geometry). The rationals and the irrationals are two totally separate number types: there is no overlap.

Putting these two major classifications, the rationals and the irrationals, together in one set gives you the "real" numbers.


FRACTIONS

Fractional numbers are ratios (divisions) of integers. In other words, a fraction is formed by dividing one integer by another integer. Set of Fraction is a subset of the set of Rational Numbers.

Fraction can be expressed in two forms fractional representation (\frac{m}{n}) and decimal representation (a.bcd).

Definition
Fractional representation
Converting Improper Fractions
Reciprocal
Operation on Fractions
Decimal Representation
Converting Decimals to Fractions
Rounding
Ratios and Proportions


POSITIVE AND NEGATIVE NUMBERS

A positive number is a real number that is greater than zero.
A negative number is a real number that is smaller than zero.

Zero is not positive, nor negative.

Multiplication:
positive * positive = positive
positive * negative = negative
negative * negative = positive

Division:
positive / positive = positive
positive / negative = negative
negative / negative = positive


EXPONENTS, ROOTS, PERCENTS

Exponents are a "shortcut" method of showing a number that was multiplied by itself several times. Roots (or radicals) are the "opposite" operation of applying exponents. A percentage is a way of expressing a number as a fraction of 100 (per cent meaning "per hundred").

Exponents
Perfect Square
Roots
Last Digit of a Product
Last Digit of a Power
Percents


ORDER OF OPERATIONS - PEMDAS

Perform the operations inside a Parenthesis first (absolute value signs also fall into this category), then Exponents, then Multiplication and Division, from left to right, then Addition and Subtraction, from left to right - PEMDAS.

Special cases:
• An exclamation mark indicates that one should compute the factorial of the term immediately to its left, before computing any of the lower-precedence operations, unless grouping symbols dictate otherwise. But 3^2! means (3^2)! = 9! while 2^{5!} = 2^{120}; a factorial in an exponent applies to the exponent, while a factorial not in the exponent applies to the entire power.

• If exponentiation is indicated by stacked symbols, the rule is to work from the top down, thus:
a^m^n=a^{(m^n)} and not (a^m)^n.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
25 extra-hard Quant Tests

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Kaplan Promo CodeKnewton GMAT Discount CodesVeritas Prep GMAT Discount Codes
Expert Post
1 KUDOS received
Math Expert
User avatar
Joined: 02 Sep 2009
Posts: 25206
Followers: 3418

Kudos [?]: 25033 [1] , given: 2702

Re: Math: Number Theory (broken into smaller topics) [#permalink] New post 10 Mar 2010, 05:21
1
This post received
KUDOS
Expert's post
INTEGERS

This post is a part of [GMAT MATH BOOK]

created by: Bunuel
edited by: bb, walker

--------------------------------------------------------
Image

The information will be included in future versions of GMAT ToolKit
[read more] [AppStore]

--------------------------------------------------------

Definition

Integers are defined as: all negative natural numbers \{...,-4, -3, -2, -1\}, zero \{0\}, and positive natural numbers \{1, 2, 3, 4, ...\}.

Note that integers do not include decimals or fractions - just whole numbers.


Even and Odd Numbers

An even number is an integer that is "evenly divisible" by 2, i.e., divisible by 2 without a remainder.
An even number is an integer of the form n=2k, where k is an integer.

An odd number is an integer that is not evenly divisible by 2.
An odd number is an integer of the form n=2k+1, where k is an integer.

Zero is an even number.

Addition / Subtraction:
even +/- even = even;
even +/- odd = odd;
odd +/- odd = even.

Multiplication:
even * even = even;
even * odd = even;
odd * odd = odd.

Division of two integers can result into an even/odd integer or a fraction.


Prime Numbers

A Prime number is a natural number with exactly two distinct natural number divisors: 1 and itself. Otherwise a number is called a composite number. Therefore, 1 is not a prime, since it only has one divisor, namely 1. A number n > 1 is prime if it cannot be written as a product of two factors a and b, both of which are greater than 1: n = ab.

• The first twenty-six prime numbers are:
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101

• Note: only positive numbers can be primes.

• There are infinitely many prime numbers.

• The only even prime number is 2, since any larger even number is divisible by 2. Also 2 is the smallest prime.

All prime numbers except 2 and 5 end in 1, 3, 7 or 9, since numbers ending in 0, 2, 4, 6 or 8 are multiples of 2 and numbers ending in 0 or 5 are multiples of 5. Similarly, all prime numbers above 3 are of the form 6n-1 or 6n+1, because all other numbers are divisible by 2 or 3.

• Any nonzero natural number n can be factored into primes, written as a product of primes or powers of primes. Moreover, this factorization is unique except for a possible reordering of the factors.

Prime factorization: every positive integer greater than 1 can be written as a product of one or more prime integers in a way which is unique. For instance integer n with three unique prime factors a, b, and c can be expressed as n=a^p*b^q*c^r, where p, q, and r are powers of a, b, and c, respectively and are \geq1.
Example: 4200=2^3*3*5^2*7.

Verifying the primality (checking whether the number is a prime) of a given number n can be done by trial division, that is to say dividing n by all integer numbers smaller than \sqrt{n}, thereby checking whether n is a multiple of m<\sqrt{n}.
Example: Verifying the primality of 161: \sqrt{161} is little less than 13, from integers from 2 to 13, 161 is divisible by 7, hence 161 is not prime.

• If n is a positive integer greater than 1, then there is always a prime number p withn < p < 2n.


Factors

A divisor of an integer n, also called a factor of n, is an integer which evenly divides n without leaving a remainder. In general, it is said m is a factor of n, for non-zero integers m and n, if there exists an integer k such that n = km.

• 1 (and -1) are divisors of every integer.

• Every integer is a divisor of itself.

• Every integer is a divisor of 0, except, by convention, 0 itself.

• Numbers divisible by 2 are called even and numbers not divisible by 2 are called odd.

• A positive divisor of n which is different from n is called a proper divisor.

• An integer n > 1 whose only proper divisor is 1 is called a prime number. Equivalently, one would say that a prime number is one which has exactly two factors: 1 and itself.

• Any positive divisor of n is a product of prime divisors of n raised to some power.

• If a number equals the sum of its proper divisors, it is said to be a perfect number.
Example: The proper divisors of 6 are 1, 2, and 3: 1+2+3=6, hence 6 is a perfect number.

There are some elementary rules:
• If a is a factor of b and a is a factor of c, then a is a factor of (b + c). In fact, a is a factor of (mb + nc) for all integers m and n.

• If a is a factor of b and b is a factor of c, then a is a factor of c.

• If a is a factor of b and b is a factor of a, then a = b or a=-b.

• If a is a factor of bc, and gcd(a,b)=1, then a is a factor of c.

• If p is a prime number and p is a factor of ab then p is a factor of a or p is a factor of b.


Finding the Number of Factors of an Integer

First make prime factorization of an integer n=a^p*b^q*c^r, where a, b, and c are prime factors of n and p, q, and r are their powers.

The number of factors of n will be expressed by the formula (p+1)(q+1)(r+1). NOTE: this will include 1 and n itself.

Example: Finding the number of all factors of 450: 450=2^1*3^2*5^2

Total number of factors of 450 including 1 and 450 itself is (1+1)*(2+1)*(2+1)=2*3*3=18 factors.


Finding the Sum of the Factors of an Integer

First make prime factorization of an integer n=a^p*b^q*c^r, where a, b, and c are prime factors of n and p, q, and r are their powers.

The sum of factors of n will be expressed by the formula: \frac{(a^{p+1}-1)*(b^{q+1}-1)*(c^{r+1}-1)}{(a-1)*(b-1)*(c-1)}

Example: Finding the sum of all factors of 450: 450=2^1*3^2*5^2

The sum of all factors of 450 is \frac{(2^{1+1}-1)*(3^{2+1}-1)*(5^{2+1}-1)}{(2-1)*(3-1)*(5-1)}=\frac{3*26*124}{1*2*4}=1209


Greatest Common Factor (Divisior) - GCF (GCD)

The greatest common divisor (gcd), also known as the greatest common factor (gcf), or highest common factor (hcf), of two or more non-zero integers, is the largest positive integer that divides the numbers without a remainder.

To find the GCF, you will need to do prime-factorization. Then, multiply the common factors (pick the lowest power of the common factors).

• Every common divisor of a and b is a divisor of gcd(a, b).
• a*b=gcd(a, b)*lcm(a, b)


Lowest Common Multiple - LCM

The lowest common multiple or lowest common multiple (lcm) or smallest common multiple of two integers a and b is the smallest positive integer that is a multiple both of a and of b. Since it is a multiple, it can be divided by a and b without a remainder. If either a or b is 0, so that there is no such positive integer, then lcm(a, b) is defined to be zero.

To find the LCM, you will need to do prime-factorization. Then multiply all the factors (pick the highest power of the common factors).


Divisibility Rules

2 - If the last digit is even, the number is divisible by 2.

3 - If the sum of the digits is divisible by 3, the number is also.

4 - If the last two digits form a number divisible by 4, the number is also.

5 - If the last digit is a 5 or a 0, the number is divisible by 5.

6 - If the number is divisible by both 3 and 2, it is also divisible by 6.

7 - Take the last digit, double it, and subtract it from the rest of the number, if the answer is divisible by 7 (including 0), then the number is divisible by 7.

8 - If the last three digits of a number are divisible by 8, then so is the whole number.

9 - If the sum of the digits is divisible by 9, so is the number.

10 - If the number ends in 0, it is divisible by 10.

11 - If you sum every second digit and then subtract all other digits and the answer is: 0, or is divisible by 11, then the number is divisible by 11.
Example: to see whether 9,488,699 is divisible by 11, sum every second digit: 4+8+9=21, then subtract the sum of other digits: 21-(9+8+6+9)=-11, -11 is divisible by 11, hence 9,488,699 is divisible by 11.

12 - If the number is divisible by both 3 and 4, it is also divisible by 12.

25 - Numbers ending with 00, 25, 50, or 75 represent numbers divisible by 25.


Factorials

Factorial of a positive integer n, denoted by n!, is the product of all positive integers less than or equal to n. For instance 5!=1*2*3*4*5.

Trailing zeros:
Trailing zeros are a sequence of 0's in the decimal representation (or more generally, in any positional representation) of a number, after which no other digits follow.

125000 has 3 trailing zeros;

The number of trailing zeros in the decimal representation of n!, the factorial of a non-negative integer n, can be determined with this formula:

\frac{n}{5}+\frac{n}{5^2}+\frac{n}{5^3}+...+\frac{n}{5^k}, where k must be chosen such that 5^k<n.

It's easier if you look at an example:

How many zeros are in the end (after which no other digits follow) of 32!?
\frac{32}{5}+\frac{32}{5^2}=6+1=7 (denominator must be less than 32, 5^2=25 is less)

Hence, there are 7 zeros in the end of 32!

The formula actually counts the number of factors 5 in n!, but since there are at least as many factors 2, this is equivalent to the number of factors 10, each of which gives one more trailing zero.

Finding the number of powers of a prime number p, in the n!.

The formula is:
\frac{n}{p}+\frac{n}{p^2}+\frac{n}{p^3} ... till p^x<n

What is the power of 2 in 25!?
\frac{25}{2}+\frac{25}{4}+\frac{25}{8}+\frac{25}{16}=12+6+3+1=22

Finding the power of non-prime in n!:

How many powers of 900 are in 50!

Make the prime factorization of the number: 900=2^2*3^2*5^2, then find the powers of these prime numbers in the n!.

Find the power of 2:
\frac{50}{2}+\frac{50}{4}+\frac{50}{8}+\frac{50}{16}+\frac{50}{32}=25+12+6+3+1=47

= 2^{47}

Find the power of 3:
\frac{50}{3}+\frac{50}{9}+\frac{50}{27}=16+5+1=22

=3^{22}

Find the power of 5:
\frac{50}{5}+\frac{50}{25}=10+2=12

=5^{12}

We need all the prime {2,3,5} to be represented twice in 900, 5 can provide us with only 6 pairs, thus there is 900 in the power of 6 in 50!.


Consecutive Integers

Consecutive integers are integers that follow one another, without skipping any integers. 7, 8, 9, and -2, -1, 0, 1, are consecutive integers.

• Sum of n consecutive integers equals the mean multiplied by the number of terms, n. Given consecutive integers \{-3, -2, -1, 0, 1,2\}, mean=\frac{-3+2}{2}=-\frac{1}{2}, (mean equals to the average of the first and last terms), so the sum equals to -\frac{1}{2}*6=-3.

• If n is odd, the sum of consecutive integers is always divisible by n. Given \{9,10,11\}, we have n=3 consecutive integers. The sum of 9+10+11=30, therefore, is divisible by 3.

• If n is even, the sum of consecutive integers is never divisible by n. Given \{9,10,11,12\}, we have n=4 consecutive integers. The sum of 9+10+11+12=42, therefore, is not divisible by 4.

• The product of n consecutive integers is always divisible by n!.
Given n=4 consecutive integers: \{3,4,5,6\}. The product of 3*4*5*6 is 360, which is divisible by 4!=24.


Evenly Spaced Set

Evenly spaced set or an arithmetic progression is a sequence of numbers such that the difference of any two successive members of the sequence is a constant. The set of integers \{9,13,17,21\} is an example of evenly spaced set. Set of consecutive integers is also an example of evenly spaced set.

• If the first term is a_1 and the common difference of successive members is d, then the n_{th} term of the sequence is given by:
a_ n=a_1+d(n-1)

• In any evenly spaced set the arithmetic mean (average) is equal to the median and can be calculated by the formula mean=median=\frac{a_1+a_n}{2}, where a_1 is the first term and a_n is the last term. Given the set \{7,11,15,19\}, mean=median=\frac{7+19}{2}=13.

• The sum of the elements in any evenly spaced set is given by:
Sum=\frac{a_1+a_n}{2}*n, the mean multiplied by the number of terms. OR, Sum=\frac{2a_1+d(n-1)}{2}*n

• Special cases:
Sum of n first integers: 1+2+...+n=\frac{1+n}{2}*n

Sum of n first odd numbers: a_1+a_2+...+a_n=1+3+...+a_n=n^2, where a_n is the last, n_{th} term and given by: a_n=2n-1. Given n=5 first odd integers, then their sum equals to 1+3+5+7+9=5^2=25.

Sum of n first positive even numbers: a_1+a_2+...+a_n=2+4+...+a_n=n(n+1), where a_n is the last, n_{th} term and given by: a_n=2n. Given n=4 first positive even integers, then their sum equals to 2+4+6+8=4(4+1)=20.

• If the evenly spaced set contains odd number of elements, the mean is the middle term, so the sum is middle term multiplied by number of terms. There are five terms in the set {1, 7, 13, 19, 25}, middle term is 13, so the sum is 13*5 =65.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
25 extra-hard Quant Tests

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Expert Post
Math Expert
User avatar
Joined: 02 Sep 2009
Posts: 25206
Followers: 3418

Kudos [?]: 25033 [0], given: 2702

Re: Math: Number Theory (broken into smaller topics) [#permalink] New post 10 Mar 2010, 05:22
Expert's post
FRACTIONS

This post is a part of [GMAT MATH BOOK]

created by: Bunuel
edited by: bb, walker

--------------------------------------------------------
Image

The information will be included in future versions of GMAT ToolKit
[read more] [AppStore]

--------------------------------------------------------

Definition

Fractional numbers are ratios (divisions) of integers. In other words, a fraction is formed by dividing one integer by another integer. Set of Fraction is a subset of the set of Rational Numbers.

Fraction can be expressed in two forms fractional representation (\frac{m}{n}) and decimal representation (a.bcd).


Fractional representation

Fractional representation is a way to express numbers that fall in between integers (note that integers can also be expressed in fractional form). A fraction expresses a part-to-whole relationship in terms of a numerator (the part) and a denominator (the whole).

• The number on top of the fraction is called numerator or nominator. The number on bottom of the fraction is called denominator. In the fraction, \frac{9}{7}, 9 is the numerator and 7 is denominator.

• Fractions that have a value between 0 and 1 are called proper fraction. The numerator is always smaller than the denominator. \frac{1}{3} is a proper fraction.

• Fractions that are greater than 1 are called improper fraction. Improper fraction can also be written as a mixed number. \frac{5}{2} is improper fraction.

• An integer combined with a proper fraction is called mixed number. 4\frac{3}{5} is a mixed number. This can also be written as an improper fraction: \frac{23}{5}


Converting Improper Fractions

• Converting Improper Fractions to Mixed Fractions:
1. Divide the numerator by the denominator
2. Write down the whole number answer
3. Then write down any remainder above the denominator
Example #1: Convert \frac{11}{4} to a mixed fraction.
Solution: Divide \frac{11}{4} = 2 with a remainder of 3. Write down the 2 and then write down the remainder 3 above the denominator 4, like this: 2\frac{3}{4}

• Converting Mixed Fractions to Improper Fractions:
1. Multiply the whole number part by the fraction's denominator
2. Add that to the numerator
3. Then write the result on top of the denominator
Example #2: Convert 3\frac{2}{5} to an improper fraction.
Solution: Multiply the whole number by the denominator: 3*5=15. Add the numerator to that: 15 + 2 = 17. Then write that down above the denominator, like this: \frac{17}{5}


Reciprocal

Reciprocal for a number x, denoted by \frac{1}{x} or x^{-1}, is a number which when multiplied by x yields 1. The reciprocal of a fraction \frac{a}{b} is \frac{b}{a}. To get the reciprocal of a number, divide 1 by the number. For example reciprocal of 3 is \frac{1}{3}, reciprocal of \frac{5}{6} is \frac{6}{5}.


Operation on Fractions

Adding/Subtracting fractions:

To add/subtract fractions with the same denominator, add the numerators and place that sum over the common denominator.

To add/subtract fractions with the different denominator, find the Least Common Denominator (LCD) of the fractions, rename the fractions to have the LCD and add/subtract the numerators of the fractions

Multiplying fractions: To multiply fractions just place the product of the numerators over the product of the denominators.

Dividing fractions: Change the divisor into its reciprocal and then multiply.

Example #1: \frac{3}{7}+\frac{2}{3}=\frac{9}{21}+\frac{14}{21}=\frac{23}{21}

Example #2: Given \frac{\frac{3}{5}}{2}, take the reciprocal of 2. The reciprocal is \frac{1}{2}. Now multiply: \frac{3}{5}*\frac{1}{2}=\frac{3}{10}.


Decimal Representation

The decimals has ten as its base. Decimals can be terminating (ending) (such as 0.78, 0.2) or repeating (recuring) decimals (such as 0.333333....).

Reduced fraction \frac{a}{b} (meaning that fraction is already reduced to its lowest term) can be expressed as terminating decimal if and only b (denominator) is of the form 2^n5^m, where m and n are non-negative integers. For example: \frac{7}{250} is a terminating decimal 0.028, as 250 (denominator) equals to 2*5^3. Fraction \frac{3}{30} is also a terminating decimal, as \frac{3}{30}=\frac{1}{10} and denominator 10=2*5.


Converting Decimals to Fractions

• To convert a terminating decimal to fraction:
1. Calculate the total numbers after decimal point
2. Remove the decimal point from the number
3. Put 1 under the denominator and annex it with "0" as many as the total in step 1
4. Reduce the fraction to its lowest terms

Example: Convert 0.56 to a fraction.
1: Total number after decimal point is 2.
2 and 3: \frac{56}{100}.
4: Reducing it to lowest terms: \frac{56}{100}=\frac{14}{25}

• To convert a recurring decimal to fraction:
1. Separate the recurring number from the decimal fraction
2. Annex denominator with "9" as many times as the length of the recurring number
3. Reduce the fraction to its lowest terms

Example #1: Convert 0.393939... to a fraction.
1: The recurring number is 39.
2: \frac{39}{99}, the number 39 is of length 2 so we have added two nines.
3: Reducing it to lowest terms: \frac{39}{99}=\frac{13}{33}.

• To convert a mixed-recurring decimal to fraction:
1. Write down the number consisting with non-repeating digits and repeating digits.
2. Subtract non-repeating number from above.
3. Divide 1-2 by the number with 9's and 0's: for every repeating digit write down a 9, and for every non-repeating digit write down a zero after 9's.

Example #2: Convert 0.2512(12) to a fraction.
1. The number consisting with non-repeating digits and repeating digits is 2512;
2. Subtract 25 (non-repeating number) from above: 2512-25=2487;
3. Divide 2487 by 9900 (two 9's as there are two digits in 12 and 2 zeros as there are two digits in 25): 2487/9900=829/3300.


Rounding

Rounding is simplifying a number to a certain place value. To round the decimal drop the extra decimal places, and if the first dropped digit is 5 or greater, round up the last digit that you keep. If the first dropped digit is 4 or smaller, round down (keep the same) the last digit that you keep.

Example:
5.3485 rounded to the nearest tenth = 5.3, since the dropped 4 is less than 5.
5.3485 rounded to the nearest hundredth = 5.35, since the dropped 8 is greater than 5.
5.3485 rounded to the nearest thousandth = 5.349, since the dropped 5 is equal to 5.


Ratios and Proportions

Given that \frac{a}{b}=\frac{c}{d}, where a, b, c and d are non-zero real numbers, we can deduce other proportions by simple Algebra. These results are often referred to by the names mentioned along each of the properties obtained.

\frac{b}{a}=\frac{d}{c} - invertendo

\frac{a}{c}=\frac{b}{d} - alternendo

\frac{a+b}{b}=\frac{c+d}{d} - componendo

\frac{a-b}{b}=\frac{c-d}{d} - dividendo

\frac{a+b}{a-b}=\frac{c+d}{c-d} - componendo & dividendo
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
25 extra-hard Quant Tests

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Expert Post
2 KUDOS received
Math Expert
User avatar
Joined: 02 Sep 2009
Posts: 25206
Followers: 3418

Kudos [?]: 25033 [2] , given: 2702

Re: Math: Number Theory (broken into smaller topics) [#permalink] New post 10 Mar 2010, 05:22
2
This post received
KUDOS
Expert's post
EXPONENTS & ROOTS

This post is a part of [GMAT MATH BOOK]

created by: Bunuel
edited by: bb, walker

--------------------------------------------------------
Image

The information will be included in future versions of GMAT ToolKit
[read more] [AppStore]

--------------------------------------------------------

EXPONENTS

Exponents are a "shortcut" method of showing a number that was multiplied by itself several times. For instance, number a multiplied n times can be written as a^n, where a represents the base, the number that is multiplied by itself n times and n represents the exponent. The exponent indicates how many times to multiple the base, a, by itself.

Exponents one and zero:
a^0=1 Any nonzero number to the power of 0 is 1.
For example: 5^0=1 and (-3)^0=1

a^1=a Any number to the power 1 is itself.

Powers of zero:
If the exponent is positive, the power of zero is zero: 0^n = 0, where n > 0.

If the exponent is negative, the power of zero (0^n, where n < 0) is undefined, because division by zero is implied.

Powers of one:
1^n=1 The integer powers of one are one.

Negative powers:
a^{-n}=\frac{1}{a^n}

Powers of minus one:
If n is an even integer, then (-1)^n=1.

If n is an odd integer, then (-1)^n =-1.

Operations involving the same exponents:
Keep the exponent, multiply or divide the bases
a^n*b^n=(ab)^n

\frac{a^n}{b^n}=(\frac{a}{b})^n

(a^m)^n=a^{mn}

a^m^n=a^{(m^n)} and not (a^m)^n

Operations involving the same bases:
Keep the base, add or subtract the exponent (add for multiplication, subtract for division)
a^n*a^m=a^{n+m}

\frac{a^n}{a^m}=a^{n-m}

Fraction as power:
a^{\frac{1}{n}}=\sqrt[n]{a}

a^{\frac{m}{n}}=\sqrt[n]{a^m}

Exponential Equations:
When solving equations with even exponents, we must consider both positive and negative possibilities for the solutions.

For instance a^2=25, the two possible solutions are 5 and -5.

When solving equations with odd exponents, we'll have only one solution.

For instance for a^3=8, solution is a=2 and for a^3=-8, solution is a=-2.

Exponents and divisibility:
a^n-b^n is ALWAYS divisible by a-b.
a^n-b^n is divisible by a+b if n is even.

a^n + b^n is divisible by a+b if n is odd, and not divisible by a+b if n is even.


Perfect Square

A perfect square, is an integer that can be written as the square of some other integer. For example 16=4^2, is an perfect square.

There are some tips about the perfect square:
• The number of distinct factors of a perfect square is ALWAYS ODD.
• The sum of distinct factors of a perfect square is ALWAYS ODD.
• A perfect square ALWAYS has an ODD number of Odd-factors, and EVEN number of Even-factors.
• Perfect square always has even number of powers of prime factors.


LAST DIGIT OF A PRODUCT

Last n digits of a product of integers are last n digits of the product of last n digits of these integers.

For instance last 2 digits of 845*9512*408*613 would be the last 2 digits of 45*12*8*13=540*104=40*4=160=60

Example: The last digit of 85945*89*58307=5*9*7=45*7=35=5?


LAST DIGIT OF A POWER

Determining the last digit of (xyz)^n:

1. Last digit of (xyz)^n is the same as that of z^n;
2. Determine the cyclicity number c of z;
3. Find the remainder r when n divided by the cyclisity;
4. When r>0, then last digit of (xyz)^n is the same as that of z^r and when r=0, then last digit of (xyz)^n is the same as that of z^c, where c is the cyclisity number.

• Integer ending with 0, 1, 5 or 6, in the integer power k>0, has the same last digit as the base.
• Integers ending with 2, 3, 7 and 8 have a cyclicity of 4.
• Integers ending with 4 (eg. (xy4)^n) have a cyclisity of 2. When n is odd (xy4)^n will end with 4 and when n is even (xy4)^n will end with 6.
• Integers ending with 9 (eg. (xy9)^n) have a cyclisity of 2. When n is odd (xy9)^n will end with 9 and when n is even (xy9)^n will end with 1.

Example: What is the last digit of 127^{39}?
Solution: Last digit of 127^{39} is the same as that of 7^{39}. Now we should determine the cyclisity of 7:

1. 7^1=7 (last digit is 7)
2. 7^2=9 (last digit is 9)
3. 7^3=3 (last digit is 3)
4. 7^4=1 (last digit is 1)
5. 7^5=7 (last digit is 7 again!)
...

So, the cyclisity of 7 is 4.

Now divide 39 (power) by 4 (cyclisity), remainder is 3.So, the last digit of 127^{39} is the same as that of the last digit of 7^{39}, is the same as that of the last digit of 7^3, which is 3.


ROOTS

Roots (or radicals) are the "opposite" operation of applying exponents. For instance x^2=16 and square root of 16=4.

General rules:
\sqrt{x}\sqrt{y}=\sqrt{xy} and \frac{\sqrt{x}}{\sqrt{y}}=\sqrt{\frac{x}{y}}.

(\sqrt{x})^n=\sqrt{x^n}

x^{\frac{1}{n}}=\sqrt[n]{x}

x^{\frac{n}{m}}=\sqrt[m]{x^n}

{\sqrt{a}}+{\sqrt{b}}\neq{\sqrt{a+b}}

\sqrt{x^2}=|x|, when x\leq{0}, then \sqrt{x^2}=-x and when x\geq{0}, then \sqrt{x^2}=x

• When the GMAT provides the square root sign for an even root, such as \sqrt{x} or \sqrt[4]{x}, then the only accepted answer is the positive root.

That is, \sqrt{25}=5, NOT +5 or -5. In contrast, the equation x^2=25 has TWO solutions, +5 and -5. Even roots have only a positive value on the GMAT.

• Odd roots will have the same sign as the base of the root. For example, \sqrt[3]{125} =5 and \sqrt[3]{-64} =-4.

• For GMAT it's good to memorize following values:
\sqrt{2}\approx{1.41}
\sqrt{3}\approx{1.73}
\sqrt{5}\approx{2.24}
\sqrt{6}\approx{2.45}
\sqrt{7}\approx{2.65}
\sqrt{8}\approx{2.83}
\sqrt{10}\approx{3.16}
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


What are GMAT Club Tests?
25 extra-hard Quant Tests

Get the best GMAT Prep Resources with GMAT Club Premium Membership

Manager
Manager
avatar
Joined: 29 Oct 2009
Posts: 203
Concentration: General Management, Sustainability
WE: Consulting (Computer Software)
Followers: 2

Kudos [?]: 56 [0], given: 12

GMAT Tests User
Re: Math: Number Theory (broken into smaller topics) [#permalink] New post 10 Mar 2010, 06:48
This is a lucky day for me :-). Thank you. +1Kudos.
Manager
Manager
User avatar
Joined: 20 Nov 2009
Posts: 170
Followers: 6

Kudos [?]: 33 [0], given: 64

GMAT Tests User
Re: Math: Number Theory (broken into smaller topics) [#permalink] New post 02 Apr 2010, 01:42
Great resource. Thanks people!
_________________

But there’s something in me that just keeps going on. I think it has something to do with tomorrow, that there is always one, and that everything can change when it comes.
http://aimingformba.blogspot.com

Intern
Intern
avatar
Joined: 29 Jun 2011
Posts: 37
Followers: 0

Kudos [?]: 2 [0], given: 0

Re: Math: Number Theory (broken into smaller topics) [#permalink] New post 20 Jul 2011, 06:52
good stuff. thank you!
Manager
Manager
User avatar
Joined: 23 Aug 2011
Posts: 84
Followers: 3

Kudos [?]: 54 [0], given: 13

Re: Math: Number Theory (broken into smaller topics) [#permalink] New post 04 Sep 2012, 20:11
Bunuel wrote:
FRACTIONS

This post is a part of [GMAT MATH BOOK]

created by: Bunuel
edited by: bb, walker

--------------------------------------------------------
Image

The information will be included in future versions of GMAT ToolKit
[read more] [AppStore]

--------------------------------------------------------

Definition

Fractional numbers are ratios (divisions) of integers. In other words, a fraction is formed by dividing one integer by another integer. Set of Fraction is a subset of the set of Rational Numbers.

Fraction can be expressed in two forms fractional representation (\frac{m}{n}) and decimal representation (a.bcd).


Fractional representation

Fractional representation is a way to express numbers that fall in between integers (note that integers can also be expressed in fractional form). A fraction expresses a part-to-whole relationship in terms of a numerator (the part) and a denominator (the whole).

• The number on top of the fraction is called numerator or nominator. The number on bottom of the fraction is called denominator. In the fraction, \frac{9}{7}, 9 is the numerator and 7 is denominator.

• Fractions that have a value between 0 and 1 are called proper fraction. The numerator is always smaller than the denominator. \frac{1}{3} is a proper fraction.

• Fractions that are greater than 1 are called improper fraction. Improper fraction can also be written as a mixed number. \frac{5}{2} is improper fraction.

• An integer combined with a proper fraction is called mixed number. 4\frac{3}{5} is a mixed number. This can also be written as an improper fraction: \frac{23}{5}


Converting Improper Fractions

• Converting Improper Fractions to Mixed Fractions:
1. Divide the numerator by the denominator
2. Write down the whole number answer
3. Then write down any remainder above the denominator
Example #1: Convert \frac{11}{4} to a mixed fraction.
Solution: Divide \frac{11}{4} = 2 with a remainder of 3. Write down the 2 and then write down the remainder 3 above the denominator 4, like this: 2\frac{3}{4}

• Converting Mixed Fractions to Improper Fractions:
1. Multiply the whole number part by the fraction's denominator
2. Add that to the numerator
3. Then write the result on top of the denominator
Example #2: Convert 3\frac{2}{5} to an improper fraction.
Solution: Multiply the whole number by the denominator: 3*5=15. Add the numerator to that: 15 + 2 = 17. Then write that down above the denominator, like this: \frac{17}{5}


Reciprocal

Reciprocal for a number x, denoted by \frac{1}{x} or x^{-1}, is a number which when multiplied by x yields 1. The reciprocal of a fraction \frac{a}{b} is \frac{b}{a}. To get the reciprocal of a number, divide 1 by the number. For example reciprocal of 3 is \frac{1}{3}, reciprocal of \frac{5}{6} is \frac{6}{5}.


Operation on Fractions

Adding/Subtracting fractions:

To add/subtract fractions with the same denominator, add the numerators and place that sum over the common denominator.

To add/subtract fractions with the different denominator, find the Least Common Denominator (LCD) of the fractions, rename the fractions to have the LCD and add/subtract the numerators of the fractions

Multiplying fractions: To multiply fractions just place the product of the numerators over the product of the denominators.

Dividing fractions: Change the divisor into its reciprocal and then multiply.

Example #1: \frac{3}{7}+\frac{2}{3}=\frac{9}{21}+\frac{14}{21}=\frac{23}{21}

Example #2: Given \frac{\frac{3}{5}}{2}, take the reciprocal of 2. The reciprocal is \frac{1}{2}. Now multiply: \frac{3}{5}*\frac{1}{2}=\frac{3}{10}.


Decimal Representation

The decimals has ten as its base. Decimals can be terminating (ending) (such as 0.78, 0.2) or repeating (recuring) decimals (such as 0.333333....).

Reduced fraction \frac{a}{b} (meaning that fraction is already reduced to its lowest term) can be expressed as terminating decimal if and only b (denominator) is of the form 2^n5^m, where m and n are non-negative integers. For example: \frac{7}{250} is a terminating decimal 0.028, as 250 (denominator) equals to 2*5^3. Fraction \frac{3}{30} is also a terminating decimal, as \frac{3}{30}=\frac{1}{10} and denominator 10=2*5.


Converting Decimals to Fractions

• To convert a terminating decimal to fraction:
1. Calculate the total numbers after decimal point
2. Remove the decimal point from the number
3. Put 1 under the denominator and annex it with "0" as many as the total in step 1
4. Reduce the fraction to its lowest terms

Example: Convert 0.56 to a fraction.
1: Total number after decimal point is 2.
2 and 3: \frac{56}{100}.
4: Reducing it to lowest terms: \frac{56}{100}=\frac{14}{25}

• To convert a recurring decimal to fraction:
1. Separate the recurring number from the decimal fraction
2. Annex denominator with "9" as many times as the length of the recurring number
3. Reduce the fraction to its lowest terms

Example #1: Convert 0.393939... to a fraction.
1: The recurring number is 39.
2: \frac{39}{99}, the number 39 is of length 2 so we have added two nines.
3: Reducing it to lowest terms: \frac{39}{99}=\frac{13}{33}.

• To convert a mixed-recurring decimal to fraction:
1. Write down the number consisting with non-repeating digits and repeating digits.
2. Subtract non-repeating number from above.
3. Divide 1-2 by the number with 9's and 0's: for every repeating digit write down a 9, and for every non-repeating digit write down a zero after 9's.

Example #2: Convert 0.2512(12) to a fraction.
1. The number consisting with non-repeating digits and repeating digits is 2512;
2. Subtract 25 (non-repeating number) from above: 2512-25=2487;
3. Divide 2487 by 9900 (two 9's as there are two digits in 12 and 2 zeros as there are two digits in 25): 2487/9900=829/3300.


Rounding

Rounding is simplifying a number to a certain place value. To round the decimal drop the extra decimal places, and if the first dropped digit is 5 or greater, round up the last digit that you keep. If the first dropped digit is 4 or smaller, round down (keep the same) the last digit that you keep.

Example:
5.3485 rounded to the nearest tenth = 5.3, since the dropped 4 is less than 5.
5.3485 rounded to the nearest hundredth = 5.35, since the dropped 8 is greater than 5.
5.3485 rounded to the nearest thousandth = 5.349, since the dropped 5 is equal to 5.


Ratios and Proportions

Given that \frac{a}{b}=\frac{c}{d}, where a, b, c and d are non-zero real numbers, we can deduce other proportions by simple Algebra. These results are often referred to by the names mentioned along each of the properties obtained.

\frac{b}{a}=\frac{d}{c} - invertendo

\frac{a}{c}=\frac{b}{d} - alternendo

\frac{a+b}{b}=\frac{c+d}{d} - componendo

\frac{a-b}{b}=\frac{c-d}{d} - dividendo

\frac{a+b}{a-b}=\frac{c+d}{c-d} - componendo & dividendo


Thanks, this a great and concise summary of all the basics.
Just to add, another important basic rule


for +ve fractions,
if \frac{a}{b}<1 and x is any positive number then \frac{(a+x)}{(b+x)}>\frac{a}{b}
,in short when the two fractions have same difference between numerator and denominator , the fraction having greater numerator is always greater.

Also, when \frac{a}{b}>1 and x is any positive number then \frac{(a+x)}{(b+x)}<\frac{a}{b}


Example which one is greater of the following two:
a)\frac{111487}{111490} b)\frac{111587}{111590} , the ans is b for the same reasons above, as \frac{111487+100}{111490+100}
_________________

Whatever one does in life is a repetition of what one has done several times in one's life!
If my post was worth it, then i deserve kudos :)

Re: Math: Number Theory (broken into smaller topics)   [#permalink] 04 Sep 2012, 20:11
    Similar topics Author Replies Last post
Similar
Topics:
33 Experts publish their posts in the topic Math: Number Theory - Percents Bunuel 43 22 Mar 2010, 14:24
392 Experts publish their posts in the topic Math: Number Theory Bunuel 147 23 Dec 2009, 17:36
NUMBER THEORY vcbabu 5 03 Feb 2009, 10:11
1 NUMBER THEORY vcbabu 2 02 Feb 2009, 10:38
PS-numbers greater smaller LM 6 24 Jun 2007, 20:03
Display posts from previous: Sort by

Math: Number Theory (broken into smaller topics)

  Question banks Downloads My Bookmarks Reviews Important topics  


cron

GMAT Club MBA Forum Home| About| Privacy Policy| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group and phpBB SEO

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.