Inverse of Subgroup

Theorem
Let $\left({G, \circ}\right)$ be a group.

Let $H$ be a subgroup of $G$.

Then:
 * $H^{-1} = H$

where $H^{-1}$ is the inverse of $H$.

Proof
As $H$ is a subgroup of $G$, $\forall h \in H: h^{-1} \in H$.

The result follows.