Ideal of Unit is Whole Ring

Theorem
Let $\struct {R, +, \circ}$ be a ring with unity.

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

If $J$ contains a unit of $R$, then $J = R$.

Proof
Let $u \in J$, where $u \in U_R$.

Also by definition, we have $u^{-1} \in U_R$.

Let $x \in R$.

Thus $R \subseteq J$.

As $J \subseteq R$ by definition, it follows that $J = R$.