15. Let P, Q, R, and S be statement forms. (a) Prove that PV ((QAR) ^ S)) and (PVQ) ^ (PVR) ^ (PVS) are equivalent statement forms. (b) Prove that PA ((QVR) v S)) and (P^Q) V (PAR) V (PAS) are equivalent statement forms. 1. Let P and Q be statement forms. Prove that Q-P is logically equivalent to P⇒ Q.