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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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    sorry, I've wrote sigma then inter in the tag and I didn't know it was measure theory :S , yes I know this formula and wt I did is subtract N from it then I got what I wrote in the question,is it right?2011-10-02
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    @Sosy: Yes, that is a correct argument. I've closed this question as a duplicate, because this question has come up before here.2011-10-02

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