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How to calculate sum of numbers in a sequence?

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by clawhammer » Sun Oct 31, 2010 1:29 am
Can anyone tell me how to calculate the SUM of numbers in a sequence,
if I know: Initial Number, Last Number, The Difference between them, and length of the sequence

For example, i want to calculate the sum of the following:
5+10+15+....+50 = ?
Here, Length = 10, Difference between 2 consecutive term = 5, starting = 5, ending =50

Then sum = ?

Can't recall this formula, need it bad. :(
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by GMATGuruNY » Sun Oct 31, 2010 2:38 am
clawhammer wrote:Can anyone tell me how to calculate the SUM of numbers in a sequence,
if I know: Initial Number, Last Number, The Difference between them, and length of the sequence

For example, i want to calculate the sum of the following:
5+10+15+....+50 = ?
Here, Length = 10, Difference between 2 consecutive term = 5, starting = 5, ending =50

Then sum = ?

Can't recall this formula, need it bad. :(
Sum = (1st + last)/2 * (number of terms)

In the sequence above:
1st term = 5
Last term = 50
Number of terms = 10
Thus, the sum = (5+50)/2 * 10 = 275.

Given the sequence 2, 5, 8, 11, 14:

Sum = (2+14)/2 * 5 = 40.
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by clawhammer » Sun Oct 31, 2010 2:46 am
Thanks Mitch.

Thought that works only for consecutive integers, seems it works for any sequence, regardless of the difference between two consecutive terms. Cool.

Thanks!
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