4. Prove each of the following by performing a derivation, starting on the left, leading to what's on the right. For each step indicate the corresponding theorem or law, as

in the marked up notes a. (А + B) · (А + В') = А+ А·В b.(X + Y)·(X + Z)-(X + Y ')=X c. А·(А + B) + А'·С = А + С