In order to prove two sets are equal, there are two methods commonly used. The first is to show that each set is a subset of the other. That is, if and then A=B. The methods of the previous section apply here. The second method is to use an algebraic approach based on Boolean Algebra. If in doubt as to which method to use, the rule of thumb is to prove each set a subset of the other. The Boolean algebra method requires knowing the axioms and elementary results. To illustrate this method, consider how to prove one of De Morgan's laws . It relies on the following result namely "If and where U is the universal set, then ". You may wish to try and prove this result from the axioms of Boolean algebra. We also make use of the more obvious results " " and " ". Anyway, let's continue with establishing De Morgan's law.