I learned the concept radical of an ideal from this wikipedia article. I tried some examples and I found that it's not easy to find $Rad(I)$. (That article gives some examples when $R={\Bbb Z}$.) For examples, let $R$ be the ring ${\Bbb Z}_n$ and let $I=\langle m\rangle $ where $m\in {\Bbb Z}_n$. How can I find $Rad(I)$? One properties I read in the article may be useful is
$Rad(I)$ is the intersection of all the prime ideals of $R$ that contain $I$.
But how can I find all the prime ideals of ${\Bbb Z}_n$ that contains $\langle m\rangle$?
I didn't find related materials in some introduction-level abstract algebra textbooks. Some happen to give this concept in exercises, say, "show that $Rad(I)$" is an ideal. Can any one come up with some useful references for this topic?