Simple maths question [closed]

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What is it called when you add numbers from 1 ..n . I know when you开发者_JAVA百科 multiply them its called factorial

1* 2 * 3 * 4* 5 = 5!

what is it called when its

1+2+3+4+5 =15?

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It seems to be called a Triangular number and it is equal to n(n+1)/2

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It's a triangular number.

http://en.wikipedia.org/wiki/Triangular_number

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you could write it in summation form

http://en.wikipedia.org/wiki/Summation

but other than summation I don't think it has a special name.

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This is named "Arithmetic progression": http://en.wikipedia.org/wiki/Arithmetic_progression

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Cumulative sum or prefix sum

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I would call it "summation from 1 to n." Which is really shorthand for "summation of i where i ranges from 1 to n".

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I don't believe it has a special name, but the numbers that result are known as the Triangluar Numbers.

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Sum of an Arithmetic Progression. You can even use a formula to calculate it quickly

s= n/2(2a+(n-1)d)

where

s = sum of progression, n = number of terms, a = value of first term, d = difference between 2 terms