Big O

Please show how to prove or displove it.

Prove or disprove the following: (a) 5n^2 - 2n + 26 elementof O(n^2). (b) forall a greaterthanorequalto 1: a^n elementof O(n!) (c) forall a greaterthanorequalto 1: 2^n + a elementof O(2^n) (d) forall a > 1: (f(n) elementof O(log_2 n)) implies (f(n) elementof O(log_a n)) (e) 2^n elementof O(n^log^2 n). (f) 2^2^n + 1 elementof O(2^2^n)