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Sequences & Progressions
This post is a part of GMAT MATH BOOK
created by: shrouded1
Definition
Sequence : It is an ordered list of objects. It can be finite or infinite. The elements may repeat themselves more than once in the sequence, and their ordering is important unlike a set
Arithmetic Progressions
Definition
It is a special type of sequence in which the difference between successive terms is constant.
General Term
\(a_n = a_{n-1} + d = a_1 + (n-1)d\)
\(a_i\) is the ith term
\(d\) is the common difference
\(a_1\) is the first term
Defining Properties
Each of the following is necessary & sufficient for a sequence to be an AP :
Summation
The sum of an infinite AP can never be finite except if \(a_1=0\) & \(d=0\)
The general sum of a n term AP with common difference d is given by \(\frac{n}{2}(2a+(n-1)d)\)
The sum formula may be re-written as \(n * Avg(a_1,a_n) = \frac{n}{2} * (FirstTerm+LastTerm)\)
Examples
Geometric Progressions
Definition
It is a special type of sequence in which the ratio of consequetive terms is constant
General Term
\(b_n = b_{n-1} * r = a_1 * r^{n-1}\)
\(b_i\) is the ith term
\(r\) is the common ratio
\(b_1\) is the first term
Defining Properties
Each of the following is necessary & sufficient for a sequence to be an GP :
Summation
The sum of an infinite GP will be finite if absolute value of r < 1
The general sum of a n term GP with common ratio r is given by \(b_1*\frac{r^n - 1}{r-1}\)
If an infinite GP is summable (|r|<1) then the sum is \(\frac{b_1}{1-r}\)
Examples
Harmonic Progressions
Definition
It is a special type of sequence in which if you take the inverse of every term, this new sequence forms an HP
Important Properties
Of any three consecutive terms of a HP, the middle one is always the harmonic mean of the other two, where the harmonic mean (HM) is defined as :
\(\frac{1}{2} * (\frac{1}{a} + \frac{1}{b}) = \frac{1}{HM(a,b)}\)
Or in other words :
\(HM(a,b) = \frac{2ab}{a+b}\)
APs, GPs, HPs : Linkage
Each progression provides us a definition of "mean" :
Arithmetic Mean : \(\frac{a+b}{2}\) OR \(\frac{a1+..+an}{n}\)
Geometric Mean : \(\sqrt{ab}\) OR \((a1 *..* an)^{\frac{1}{n}}\)
Harmonic Mean : \(\frac{2ab}{a+b}\) OR \(\frac{n}{\frac{1}{a1}+..+\frac{1}{an}}\)
For all non-negative real numbers : AM >= GM >= HM
In particular for 2 numbers : AM * HM = GM * GM
Example :
Let a=50 and b=2,
then the AM = (50+2)*0.5 = 26 ;
the GM = sqrt(50*2) = 10 ;
the HM = (2*50*2)/(52) = 3.85
AM > GM > HM
AM*HM = 100 = GM^2
Misc Notes
A subsequence (any set of consequutive terms) of an AP is an AP
A subsequence (any set of consequutive terms) of a GP is a GP
A subsequence (any set of consequutive terms) of a HP is a HP
If given an AP, and I pick out a subsequence from that AP, consisting of the terms \(a_{i1},a_{i2},a_{i3},...\) such that \(i1,i2,i3\) are in AP then the new subsequence will also be an AP
For Example : Consider the AP with \(a_1=1, d=2\) {1,3,5,7,9,11,...}, so a_n=1+2*(n-1)=2n-1
Pick out the subsequence of terms \(a_5,a_{10},a_{15},...\)
New sequence is {9,19,29,...} which is an AP with \(a_1=9\) and \(d=10\)
If given a GP, and I pick out a subsequence from that GP, consisting of the terms \(b_{i1},b_{i2},b_{i3},...\) such that \(i1,i2,i3\) are in AP then the new subsequence will also be a GP
For Example : Consider the GP with \(b_1=1, r=2\) {1,2,4,8,16,32,...}, so b_n=2^(n-1)
Pick out the subsequence of terms \(b_2,b_4,b_6,...\)
New sequence is {4,16,64,...} which is a GP with \(b_1=4\) and \(r=4\)
The special sequence in which each term is the sum of previous two terms is known as the fibonacci sequence. It is neither an AP nor a GP. The first two terms are 1. {1,1,2,3,5,8,13,...}
In a finite AP, the mean of all the terms is equal to the mean of the middle two terms if n is even and the middle term if n is odd. In either case this is also equal to the mean of the first and last terms
Some examples
Example 1
A coin is tossed repeatedly till the result is a tails, what is the probability that the total number of tosses is less than or equal to 5 ?
Solution
P(<=5 tosses) = P(1 toss)+...+P(5 tosses) = P(T)+P(HT)+P(HHT)+P(HHHT)+P(HHHHT)
We know that P(H)=P(T)=0.5
So Probability = 0.5 + 0.5^2 + ... + 0.5^5
This is just a finite GP, with first term = 0.5, n=5 and ratio = 0.5. Hence :
Probability = \(0.5 * \frac{1-0.5^5}{1-0.5} = \frac{1}{2} * \frac{\frac{31}{32}}{\frac{1}{2}} = \frac{31}{32}\)
Example 2
In an arithmetic progression a1,a2,...,a22,a23, the common difference is non-zero, how many terms are greater than 24 ?
(1) a1 = 8
(2) a12 = 24
Solution
(1) a1=8, does not tell us anything about the common difference, so impossible to say how many terms are greater than 24
(2) a12=24, and we know common difference is non-zero. So either all the terms below a12 are greater than 24 and the terms above it less than 24 or the other way around. In either case, there are exactly 11 terms either side of a12. Sufficient
Answer is B
Example 3
For positive integers a,b (a<b) arrange in ascending order the quantities a, b, sqrt(ab), avg(a,b), 2ab/(a+b)
Solution
Using the inequality AM>=GM>=HM, the solution is :
a <= 2ab/(a+b) <= Sqrt(ab) <= Avg(a,b) <= b
Example 4
For every integer k from 1 to 10, inclusive, the kth term of a certain sequence is given by (-1)^(k+1) *(1/2^k). If T is the sum of the first 10 terms in the sequence then T is
a)greater than 2
b)between 1 and 2
c)between 1/2 and 1
d)between 1/4 and 1/2
e)less than 1/4.
Solution
The sequence given has first term 1/2 and each subsequent term can be obtained by multiplying with -1/2. So it is a GP. We can use the GP summation formula
\(S=b\frac{1-r^n}{1-r}=\frac{1}{2} * \frac{1-(-1/2)^{10}}{1-(-1/2)} = \frac{1}{3} * \frac{1023}{1024}\)
1023/1024 is very close to 1, so this sum is very close to 1/3
Answer is d
Example 5
The sum of the fourth and twelfth term of an arithmetic progression is 20. What is the sum of the first 15 terms of the arithmetic progression?
A. 300
B. 120
C. 150
D. 170
E. 270
Solution
\(a_4+a_12=20\)
\(a_4=a_1+3d, a_12=a_1+11d\)
\(2a_1+14d=20\)
Now we need the sum of first 15 terms, which is given by :
\(\frac{15}{2} (2a_1 + (15-1)d) = \frac{15}{2} * (2a_1+14d) = 150\)
Answer is (c)
Practice Questions
Easy:
1. https://gmatclub.com/forum/the-infinite ... 00090.html
2. https://gmatclub.com/forum/in-an-increa ... 43749.html
3. https://gmatclub.com/forum/the-sequence ... 46630.html
4. https://gmatclub.com/forum/the-sequence ... 34498.html
5. https://gmatclub.com/forum/in-a-certain ... 44376.html
6. https://gmatclub.com/forum/what-is-the- ... 44459.html
7. https://gmatclub.com/forum/in-the-infin ... 93065.html
8. https://gmatclub.com/forum/the-sequence ... 66830.html
9. https://gmatclub.com/forum/in-the-seque ... 98247.html
10. https://gmatclub.com/forum/for-any-sequ ... 28100.html
Medium:
1. https://gmatclub.com/forum/in-a-certain ... 19235.html
2. https://gmatclub.com/forum/in-the-seque ... 68359.html
3. https://gmatclub.com/forum/what-is-the- ... 30041.html
4. https://gmatclub.com/forum/in-the-seque ... 98182.html
5. https://gmatclub.com/forum/a-sequence-i ... 71548.html
6. https://gmatclub.com/forum/the-sequence ... 60525.html
7. https://gmatclub.com/forum/if-the-sum-o ... 98380.html
8. https://gmatclub.com/forum/if-s1-s2-s3- ... 02927.html
9. https://gmatclub.com/forum/around-the-w ... 16033.html
10. https://gmatclub.com/forum/gmat-club-wo ... 95168.html
Hard:
1. https://gmatclub.com/forum/a-sequence-o ... 20319.html
2. https://gmatclub.com/forum/a-certain-mu ... -3940.html
3. https://gmatclub.com/forum/the-sequence ... 27175.html
4. https://gmatclub.com/forum/if-the-sum-o ... 98379.html
5. https://gmatclub.com/forum/the-sum-of-n ... 00640.html
6. https://gmatclub.com/forum/x-y-z-is-an- ... 98430.html
7. https://gmatclub.com/forum/gmat-club-ol ... 67365.html
8. https://gmatclub.com/forum/if-the-sum-o ... 98381.html
9. https://gmatclub.com/forum/around-the-w ... 16261.html
10. https://gmatclub.com/forum/around-the-w ... 16134.html
_________________
created by: shrouded1
Definition
Sequence : It is an ordered list of objects. It can be finite or infinite. The elements may repeat themselves more than once in the sequence, and their ordering is important unlike a set
Arithmetic Progressions
Definition
It is a special type of sequence in which the difference between successive terms is constant.
General Term
\(a_n = a_{n-1} + d = a_1 + (n-1)d\)
\(a_i\) is the ith term
\(d\) is the common difference
\(a_1\) is the first term
Defining Properties
Each of the following is necessary & sufficient for a sequence to be an AP :
- \(a_i - a_{i-1} =\) Constant
- If you pick any 3 consecutive terms, the middle one is the mean of the other two
- For all i,j > k >= 1 : \(\frac{a_i - a_k}{i-k} = \frac{a_j-a_k}{j-k}\)
Summation
The sum of an infinite AP can never be finite except if \(a_1=0\) & \(d=0\)
The general sum of a n term AP with common difference d is given by \(\frac{n}{2}(2a+(n-1)d)\)
The sum formula may be re-written as \(n * Avg(a_1,a_n) = \frac{n}{2} * (FirstTerm+LastTerm)\)
Examples
- All odd positive integers : {1,3,5,7,...} \(a_1=1, d=2\)
- All positive multiples of 23 : {23,46,69,92,...} \(a_1=23, d=23\)
- All negative reals with decimal part 0.1 : {-0.1,-1.1,-2.1,-3.1,...} \(a_1=-0.1, d=-1\)
Geometric Progressions
Definition
It is a special type of sequence in which the ratio of consequetive terms is constant
General Term
\(b_n = b_{n-1} * r = a_1 * r^{n-1}\)
\(b_i\) is the ith term
\(r\) is the common ratio
\(b_1\) is the first term
Defining Properties
Each of the following is necessary & sufficient for a sequence to be an GP :
- \(\frac{b_i}{b_{i-1}} =\) Constant
- If you pick any 3 consecutive terms, the middle one is the geometric mean of the other two
- For all i,j > k >= 1 : \((\frac{b_i}{b_k})^{j-k} = (\frac{b_j}{b_k})^{i-k}\)
Summation
The sum of an infinite GP will be finite if absolute value of r < 1
The general sum of a n term GP with common ratio r is given by \(b_1*\frac{r^n - 1}{r-1}\)
If an infinite GP is summable (|r|<1) then the sum is \(\frac{b_1}{1-r}\)
Examples
- All positive powers of 2 : {1,2,4,8,...} \(b_1=1, r=2\)
- All negative powers of 4 : {1/4,1/16,1/64,1/256,...} \(b_1=1/4, r=1/4, sum=\frac{1/4}{(1-1/4)}=(1/3)\)
Harmonic Progressions
Definition
It is a special type of sequence in which if you take the inverse of every term, this new sequence forms an HP
Important Properties
Of any three consecutive terms of a HP, the middle one is always the harmonic mean of the other two, where the harmonic mean (HM) is defined as :
\(\frac{1}{2} * (\frac{1}{a} + \frac{1}{b}) = \frac{1}{HM(a,b)}\)
Or in other words :
\(HM(a,b) = \frac{2ab}{a+b}\)
APs, GPs, HPs : Linkage
Each progression provides us a definition of "mean" :
Arithmetic Mean : \(\frac{a+b}{2}\) OR \(\frac{a1+..+an}{n}\)
Geometric Mean : \(\sqrt{ab}\) OR \((a1 *..* an)^{\frac{1}{n}}\)
Harmonic Mean : \(\frac{2ab}{a+b}\) OR \(\frac{n}{\frac{1}{a1}+..+\frac{1}{an}}\)
For all non-negative real numbers : AM >= GM >= HM
In particular for 2 numbers : AM * HM = GM * GM
Example :
Let a=50 and b=2,
then the AM = (50+2)*0.5 = 26 ;
the GM = sqrt(50*2) = 10 ;
the HM = (2*50*2)/(52) = 3.85
AM > GM > HM
AM*HM = 100 = GM^2
Misc Notes
A subsequence (any set of consequutive terms) of an AP is an AP
A subsequence (any set of consequutive terms) of a GP is a GP
A subsequence (any set of consequutive terms) of a HP is a HP
If given an AP, and I pick out a subsequence from that AP, consisting of the terms \(a_{i1},a_{i2},a_{i3},...\) such that \(i1,i2,i3\) are in AP then the new subsequence will also be an AP
For Example : Consider the AP with \(a_1=1, d=2\) {1,3,5,7,9,11,...}, so a_n=1+2*(n-1)=2n-1
Pick out the subsequence of terms \(a_5,a_{10},a_{15},...\)
New sequence is {9,19,29,...} which is an AP with \(a_1=9\) and \(d=10\)
If given a GP, and I pick out a subsequence from that GP, consisting of the terms \(b_{i1},b_{i2},b_{i3},...\) such that \(i1,i2,i3\) are in AP then the new subsequence will also be a GP
For Example : Consider the GP with \(b_1=1, r=2\) {1,2,4,8,16,32,...}, so b_n=2^(n-1)
Pick out the subsequence of terms \(b_2,b_4,b_6,...\)
New sequence is {4,16,64,...} which is a GP with \(b_1=4\) and \(r=4\)
The special sequence in which each term is the sum of previous two terms is known as the fibonacci sequence. It is neither an AP nor a GP. The first two terms are 1. {1,1,2,3,5,8,13,...}
In a finite AP, the mean of all the terms is equal to the mean of the middle two terms if n is even and the middle term if n is odd. In either case this is also equal to the mean of the first and last terms
Some examples
Example 1
A coin is tossed repeatedly till the result is a tails, what is the probability that the total number of tosses is less than or equal to 5 ?
Solution
P(<=5 tosses) = P(1 toss)+...+P(5 tosses) = P(T)+P(HT)+P(HHT)+P(HHHT)+P(HHHHT)
We know that P(H)=P(T)=0.5
So Probability = 0.5 + 0.5^2 + ... + 0.5^5
This is just a finite GP, with first term = 0.5, n=5 and ratio = 0.5. Hence :
Probability = \(0.5 * \frac{1-0.5^5}{1-0.5} = \frac{1}{2} * \frac{\frac{31}{32}}{\frac{1}{2}} = \frac{31}{32}\)
Example 2
In an arithmetic progression a1,a2,...,a22,a23, the common difference is non-zero, how many terms are greater than 24 ?
(1) a1 = 8
(2) a12 = 24
Solution
(1) a1=8, does not tell us anything about the common difference, so impossible to say how many terms are greater than 24
(2) a12=24, and we know common difference is non-zero. So either all the terms below a12 are greater than 24 and the terms above it less than 24 or the other way around. In either case, there are exactly 11 terms either side of a12. Sufficient
Answer is B
Example 3
For positive integers a,b (a<b) arrange in ascending order the quantities a, b, sqrt(ab), avg(a,b), 2ab/(a+b)
Solution
Using the inequality AM>=GM>=HM, the solution is :
a <= 2ab/(a+b) <= Sqrt(ab) <= Avg(a,b) <= b
Example 4
For every integer k from 1 to 10, inclusive, the kth term of a certain sequence is given by (-1)^(k+1) *(1/2^k). If T is the sum of the first 10 terms in the sequence then T is
a)greater than 2
b)between 1 and 2
c)between 1/2 and 1
d)between 1/4 and 1/2
e)less than 1/4.
Solution
The sequence given has first term 1/2 and each subsequent term can be obtained by multiplying with -1/2. So it is a GP. We can use the GP summation formula
\(S=b\frac{1-r^n}{1-r}=\frac{1}{2} * \frac{1-(-1/2)^{10}}{1-(-1/2)} = \frac{1}{3} * \frac{1023}{1024}\)
1023/1024 is very close to 1, so this sum is very close to 1/3
Answer is d
Example 5
The sum of the fourth and twelfth term of an arithmetic progression is 20. What is the sum of the first 15 terms of the arithmetic progression?
A. 300
B. 120
C. 150
D. 170
E. 270
Solution
\(a_4+a_12=20\)
\(a_4=a_1+3d, a_12=a_1+11d\)
\(2a_1+14d=20\)
Now we need the sum of first 15 terms, which is given by :
\(\frac{15}{2} (2a_1 + (15-1)d) = \frac{15}{2} * (2a_1+14d) = 150\)
Answer is (c)
Practice Questions
Easy:
1. https://gmatclub.com/forum/the-infinite ... 00090.html
2. https://gmatclub.com/forum/in-an-increa ... 43749.html
3. https://gmatclub.com/forum/the-sequence ... 46630.html
4. https://gmatclub.com/forum/the-sequence ... 34498.html
5. https://gmatclub.com/forum/in-a-certain ... 44376.html
6. https://gmatclub.com/forum/what-is-the- ... 44459.html
7. https://gmatclub.com/forum/in-the-infin ... 93065.html
8. https://gmatclub.com/forum/the-sequence ... 66830.html
9. https://gmatclub.com/forum/in-the-seque ... 98247.html
10. https://gmatclub.com/forum/for-any-sequ ... 28100.html
Medium:
1. https://gmatclub.com/forum/in-a-certain ... 19235.html
2. https://gmatclub.com/forum/in-the-seque ... 68359.html
3. https://gmatclub.com/forum/what-is-the- ... 30041.html
4. https://gmatclub.com/forum/in-the-seque ... 98182.html
5. https://gmatclub.com/forum/a-sequence-i ... 71548.html
6. https://gmatclub.com/forum/the-sequence ... 60525.html
7. https://gmatclub.com/forum/if-the-sum-o ... 98380.html
8. https://gmatclub.com/forum/if-s1-s2-s3- ... 02927.html
9. https://gmatclub.com/forum/around-the-w ... 16033.html
10. https://gmatclub.com/forum/gmat-club-wo ... 95168.html
Hard:
1. https://gmatclub.com/forum/a-sequence-o ... 20319.html
2. https://gmatclub.com/forum/a-certain-mu ... -3940.html
3. https://gmatclub.com/forum/the-sequence ... 27175.html
4. https://gmatclub.com/forum/if-the-sum-o ... 98379.html
5. https://gmatclub.com/forum/the-sum-of-n ... 00640.html
6. https://gmatclub.com/forum/x-y-z-is-an- ... 98430.html
7. https://gmatclub.com/forum/gmat-club-ol ... 67365.html
8. https://gmatclub.com/forum/if-the-sum-o ... 98381.html
9. https://gmatclub.com/forum/around-the-w ... 16261.html
10. https://gmatclub.com/forum/around-the-w ... 16134.html
_________________
gurpreetsingh
Joined: 12 Oct 2009
Last visit: 15 Jun 2019
Posts: 2,266
Status:<strong>Nothing comes easy: neither do I want.</strong>
Location: Malaysia
Concentration: Technology, Entrepreneurship
Schools: ISB '15 (M)
GMAT 1: 670 Q49 V31
GMAT 2: 710 Q50 V35
Products:
Great initiative.
Suggestions :
1. Include AM, GM , HM included between 2 numbers.
2. use a_{n} with math tag to get \(a_{n}\)
3. Include more examples.
This post,along with the algebra, is a good initiative to fill the important topics of Math Book that were not included earlier.
Suggestions :
1. Include AM, GM , HM included between 2 numbers.
2. use a_{n} with math tag to get \(a_{n}\)
3. Include more examples.
This post,along with the algebra, is a good initiative to fill the important topics of Math Book that were not included earlier.
General Discussion
Thanks for putting it here!
gurpreetsingh
Check
Check
Check
Let me know what else ?
_________________
ezhilkumarank
Joined: 24 Jun 2010
Last visit: 08 May 2014
Posts: 270
Status:Time to step up the tempo
Location: Milky way
Schools:ISB, Tepper - CMU, Chicago Booth, LSB
Posts: 270
Great initiative. +1 to you.
Great one 
Kudos to you !
Kudos to you !
added a couple more solved GMAT style questions to the end
_________________
_________________
Posts: 379
Good to know. It can definitely same valuable time.
Posts: 150
This is great post but I was just wondering whether we need to know these concepts for GMAT
There are several questions in the OG that use these concepts. So I think its good to know all this. Plus if you search through the forums you'll find several Qs on sequences and progressions as well
_________________
_________________
Valuable resource.
Kudos +1
Kudos +1
Posts: 1
I had a question. I am not sure if this works: "In case of n numbers : AM * HM = GM^n".
This seems to work for 2 numbers but for more than 2, it seems to break, please let me know if I am missing something.
This seems to work for 2 numbers but for more than 2, it seems to break, please let me know if I am missing something.
nitantsharma
You are correct, this should only hold for special case n=2. Thanks for pointing out
_________________
Posts: 513
How can we get the formular for sumation of GEOMETRIC PROGRESSION. Please, prove, so that I do not have to remember the formular but to know the way to get the formular and so can solve the relative questions.
! |
This proof is beyond the scope of the GMAT |
The proof below is based on mathematical induction
To prove : The sum of an n term GP : \(b,br,br^2,...,br^{n-1}\) is \(b*\frac{r^n-1}{r-1}\)
P(1 term) : The sum of the GP {b} is \(b*\frac{r^1-1}{r-1}=b\). Which is true trivially
P(n terms) : Let the sum of an n term GP : \(b,br,...,br^{n-1}\) be \(b*\frac{r^n-1}{r-1}\)
P(n+1 terms) : Consider the n+1 term GP : \(b,br,....,br^n\)
Sum of this GP = Sum of n term GP + \(br^n\) = \(b*\frac{r^n-1}{r-1} + br^n\)
Sum = \(\frac{b}{r-1} * (r^n - 1 + r^n(r-1))\)
=\(\frac{b}{r-1} *(r^{n+1}-1)\)
Hence P(1) is true
And if we assume P(n) true P(n+1) is true
By mathematical induction P(k) must be true for all k>=1
Hence, proved
_________________
Can someone please clarify:
I think the formula for calculating the sum of n consecutive numbers should be:
\((lastterm - firstterm)*(lastterm - firstterm + 1)/2\)
I think the formula for calculating the sum of n consecutive numbers should be:
\((lastterm - firstterm)*(lastterm - firstterm + 1)/2\)
nonameee
1,2,3,4,5
let's apply your formula on the above series.
\(\frac{(5-1)*(5-1+1)}{2} = \frac{4*5}{2} = 10\)
this is not correct. let's try another formula.
\(\frac{n}{2}(firstterm + lastterm)\)
\(\frac{5}{2}(5+1) = 15\)
\(\frac{1}{2}(firstterm + lastterm)\) basically gives you the avg of the series. when you multiply the avg with number of terms (n), you get the sum.
HTH
dimitri92, thanks. I must have made a computational error.
i know i read it somewhere, but i cant find it now. What are the ways to get the full Math book?
Schools: Booth '15 (A) Stern '15 (A)
Posts: 5
| The general sum of a n term GP with common ratio r is given by |
For sum of a GP:
When r > 1, the denominator is (r-1)
When r < 1, the denominator is (1-r)
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