Consider the following claim and the first sentence of a proof of the claim:
Claim: If m and n are integers such that mn is even, then m is even or n is even.
Proof: Suppose that m and n are even integers and mn is odd. Is this the beginning of
(a) a direct proof
(b) a proof by contradiction
(c) a disproof by giving a counterexample
(d) another type of proof or disproof
Claim: If m and n are integers such that mn is even, then m is even or n is even.
Proof: Suppose that m and n are even integers and mn is odd. Is this the beginning of
(a) a direct proof
(b) a proof by contradiction
(c) a disproof by giving a counterexample
(d) another type of proof or disproof
总之一定不是 b 。
