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Possible Duplicate:
Proof for formula for sum of sequence 1+2+3+…+n?

I have this sigma:$\sum_{i=1}^{N}(i-1)$

is it $\frac{n^2-n}{2}\quad?$

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    @Sosy: Yes, that is a correct ar$g$ument. I've closed this question $a$s $a$ duplicate, because this questio$n$ has come up before here.2011-10-02

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There is an elementary proof that $\sum_{i = 1}^n i = \frac{n(n+1)}{2}$, which legend has is due to Gauss. For a proof, see my blog post at Math ∩ Programming.

What you have is the same as $\sum_{i = 1}^{N-1} i$, since adding zero is trivial. So your thing is correct, and substituting $n$ for $N-1$ we get $N(N-1)/2$, which is what you have.

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    Right. I meant the particular proof is due to Gauss (which might also be untrue).2011-10-02