So basically you took the formula ##1+x+\cdots+x^{k1}=\frac{1x^{k}}{1x}## (which doesn't hold for x=1 by the way) and just multiplied it by x1? 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 righthand 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)?
