I know that the rule is that

sigma k with n on top of it and k=1 on bottom = 1/2 n(n+1)

sorry if that is hard to understand but i dont know how to type sigma notation on here

but i had a problem in one of my calc. problems where it was sigma (k-1) with n on top and bottom as k=1, the answer is 1/2n(n-1)

I don't understand where the -1 comes from ??? It makes no sense to me

I tried the problem, I just don't understand it and I have a test tomorrow... please help...thank you