View Single Post
Feb11-12, 05:26 AM
Sci Advisor
PF Gold
Fredrik's Avatar
P: 9,340
So basically you took the formula ##1+x+\cdots+x^{k-1}=\frac{1-x^{k}}{1-x}## (which doesn't hold for x=1 by the way) and just multiplied it by x-1? If I was your teacher, I wouldn't accept that as an answer. (The formula you found is too similar to what you're supposed to prove). Why don't you just do the multiplication on your right-hand side to see what you get? Do you know how to evaluate a(b+c)? How about (a+b)(b+c)? How about (a+b)(b+c+d)?