(r^(n+1)-1)/(r-1) = the sum (between k=0 and n) of r^k = Sn (The n in Sn is lower than the S)
he hints to multiply by r-1 or use long division
then I'm supposed to use part to show why
the sum K=0 to infinity = (1)/(1-r)
i can graph the sequences and I know its true but I don't understand what to do tomake it work.