Radical Ideal iff Quotient Ring is Reduced

Theorem
Let $\left({R, +, \circ}\right)$ be a commutative ring with unity.

Let $J$ be an ideal of $R$.

Then $J$ is a radical ideal the quotient ring $R / J$ is a reduced ring.

Proof
Since $J \subset R$, it follows from:
 * Quotient Ring of Commutative Ring is Commutative

and:
 * Quotient Ring of Ring with Unity is Ring with Unity

that $R / J$ is a commutative ring with unity.

Let $0_{R / J}$ be the zero of $R / J$.

Sufficient Condition
Let $J$ be radical.

We need to show that:
 * if $x + J \in R / J$ such that $\left({x + J}\right)^n = 0_{R / J}$ for some positive integer $n$
 * then $x + J = 0_{R / J}$.

Let $\left({x + J}\right)^n = 0_{R / J}$.

Then:
 * $x^n + J = 0_{R / J}$

and therefore:
 * $x^n \in J$

Because $J$ is radical it follows that:
 * $x \in J$

That is:
 * $x + J = 0_{R / J}$

Therefore $R / J$ is reduced.

Necessary Condition
Let $R / J$ be reduced.

Let $x \in R$ be such that $x^n \in J$.

Then:
 * $0_{R / J} = J = x^n + J = \left({x + J}\right)^n$

Because $A / J$ is reduced, this implies that:
 * $x + J = 0_{R / J}$

That is:
 * $x \in J$

This shows that $J$ is radical.