(a) Evaluate [a_, a+], where at are the raising and lowering operators. (b) Proof the following commutator identities for opterators A, B, C: [A, BC = [A, B C + BAC]. [A, [ B. C] + [B, [C, A] + [C, [A. B] = 0. (c) Assume two operators A, B satisfy B, A, B]] = [A, [4. 8]] -0. Show A, B" = nBn-1 A. B