Since the disproof
of a proposition p ⇒ q is merely a proof of the proposition ~ (p ⇒ q) Hence, this is
also a method of proof Example 8 For each n,
22
1
n + is a prime (n ∈ N) This was once thought to be true on the basis that
212
1
+  = 22 +  1 = 5 is a prime