6.7 Let and be any Boolean formulas. (a) Consider the Boolean formula .pt NOT(p). Prove or refute that is a tautology. (b) Consider the Boolean formula = ( ) (NOT(p) + a). Prove or refute that is a tautology. 6.10 Let = (X(Y + Z)), and let b (XY + X . Z). Prove that and are logically equivalent. 6.12 Definition 6.16 Let p BF, then p is a contradiction if T(p) = 0 for every assignment T Prove the following claim. Lemma 6.15 p is a contradiction (-p) is a tautology