Past exam of the mathematics course of the University of Cambridge / 2017 / ib / Paper 2 / 11E / b

Consider all ideals containing $I$ and disjoint from the nonempty multiplicative subset $S$. This collection is nonempty because it contains $I$. The ascending chain condition for the Noetherian ring gives a maximal member $P$: otherwise repeatedly choosing a strictly larger member would give an infinite strictly ascending chain of ideals. Since $S$ is nonempty and $P$ misses it, $P\ne R$.

Suppose $ab\in P$ but $a,b\notin P$. Maximality implies that $P+(a)$ and $P+(b)$ both meet $S$, so write $s=p+ra\in S$ and $t=q+ub\in S$ with $p,q\in P$. Thenwhile closure under products gives $st\in S$, a contradiction. HenceThis proves the prime ideal avoiding a multiplicative subset result without needing $1\in S$. The assumptions themselves rule out $0\in S$, since every ideal contains zero.