prove that sigma(1/r(r+1)(r+2)=0.25-1/2(n+1)(n+2)

it is the proving for k+1 i have a question about, do you put both sides equal to each other and the expand/simplify one side using the fact that you have assumed it is true for k and the show that both sides equal each other. my teacher went over this recently however i belive she got it wrong, any help would be greatly appreciated.