Proof:
We claim that the ideal <x,2> subset Z[x] is not principal. Suppose, to the contrary, that <x,2> = <p(x)> for some p(x) in Z[x]. p(x) cannot be a constant polynomial, because this constant would have to be even, and then x is not in <p(x)>. Furthermore, p(x) has a nonzero constant term, since otherwise 2 is not in <p(x)>.
Since x is in <p(x)>, there exists a polynomial q(x) in Z[x] such that x = q(x)p(x), and since the constant term of q(x)p(x) is zero, q(x) has degree at least 1. But this means x = q(x)p(x) has degree at least 2, a contradiction.
