"Let , then . If we are done, so assume that . Clearly then, we have that , but since it follows that . We may therefore conclude that or and thus . Conversely, suppose that . Then, or so that or . This leads us to conclude that or , Using the distributive properties of conjunction (and) and disjunction (or) we may therefore conclude that and since we see that the above implies ."
Alternatively, you may just use the algebra of sets.