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