Prove that
(a) The sentence $P ↔ Q$ has the same truth table as $(P → Q) ∧ (Q → P)$.
(b) The neg statement $\neg (\neg P)$ has the same truth table as $P$, and $\neg (\neg (\neg P))$ than $\neg P$.
(c) The sentence $P → Q$ has the same truth table as $\neg P ∨ Q$.