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Re: Quick tips for adding numbers x to y [#permalink]
10 Jun 2009, 21:58

1) Add the numbers from 40 to 70, inclusive. Formula for sum of first N natural numbers is = N(N+1)/2 Sum from 1 to 70 Sa= 70*71/2 Sum from 1 to 40 Sb= 40*41/2 So sum from 40 to 70 = Sa-Sb

2) Add the even numbers from 40 to 70, inclusive. 3) Add the odd numbers from 40 to 70, inclusive.

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As a general solution to all these kind of problems learn the AP series. google on Arithemic Progression series. It is kind of difficult to write the formula here but all these calculations are tooooooo simple using this series.

Re: Quick tips for adding numbers x to y [#permalink]
12 Jun 2009, 13:34

3

This post received KUDOS

If you don't mind remembering a formula or two then yes you can. The examples that you have given can be grouped under Arithmetic Progressions...or a finite sequence of evenly spaced numbers.

There are two formulas:

1. S = \frac{n}{2} [2a + (n-1)d]

where S=\text{sum of the all the numbers in the sequence} n=\text{total number of numbers in the sequence} a=\text{the first number of the sequence} d=\text{the different between any two consecutive numbers in the sequence}

2. S = \frac{n}{2} [\text{First Term}+\text{Last Term}]

where S=\text{sum of the all the numbers in the sequence} n=\text{total number of numbers in the sequence}

You can use either equation based on what is provided in the question.

Lets take your examples and solve them. For all of these we know the first and the last number so we should be fine with equation 2.

1) Add the numbers from 40 to 70, inclusive. Solution: Here the sequence is 40,41, 42, ...., 69, 70. To calculate n n=\text{Last number}-\text{First number} + 1

n=70-40+1 = 31

Using equation 1.

S = \frac{n}{2} [\text{First Term}+\text{Last Term}]

S = \frac{n}{2} [\text{First Term}+\text{Last Term}]

S = \frac{31}{2} [40+70]

S = \frac{31}{2} [110]

S = 31\times55

S = 1705

2) Add the even numbers from 40 to 70, inclusive. Here the sequence is 40,42, 44, ...., 68, 70. Solve using 40 as the first term and 70 as the last term To calculate n for even (or odd) number