I wanted to post this in the homework forum, but there's only pre-calc for math. Question: Show that i*r^i-(i-1)*r^(i-1) = r^(i-1)-(1-r)i*r^i-1. Use this result to find the sum of i*r^(i-1) from i=1 to i=n. I've done the first part of this question, but need some help with the second. Thanks!