First show that the identity is in H. Then assume that <i>a</i> and 
<i>b</i> 
satisfy the
condition that defines the elements of H. Use this assumption to show
that<i> ab</i> satisfies the condition that defines H. Finally, use the
assumption that <i>a</i> satisfies the condition that defines H to show 
that <i>a<sup>-1</sup></i> satisfies the condition that defines H.