Exercise 1. Write a natural-deduction ₁ (p^q^r) → (PV¬qV¬r) This is a more general version of de Morgan's law (4). 2. Write a natural-deduction proof of the most general de Morgan's

law (4): 42 = (P₁ ^^ Pn) → (P₁ VV-pn) where n > 2. proof of the following WFF: