Theorem 4.14 (Zero Factor). If ab = 0, then a = 0 or b = 0.
Proof by Contradiction. Suppose both a and b are nonzero. Then say a must
have a multiplicative inverse a−1. Then,
a−1ab = a−10 (34)
b = 0. (35)
But by assumption b 6= 0. Thus a contradiction is reached, and the assumption
is false. Thus both a and b cannot be nonzero, or at least one of them is zero.