5. Let
A =
a ,
a ≠ 0. If
a is a unit then |
R/
A| = 1. Otherwise, by Theorem 4 §3.3, let
B/
A be any ideal of
R/
A, say
B =
b. Then
a
⊆
b so
b|
a. Since
a has a prime factorization, there are at most finitely many such divisors
b of
a up to associates, and hence only finitely ...