Similarly, the second and fourth sets are the same since they both contain exactly the same elements a, c, d, e. The order in which these elements are listed is different in the two sets and there are repetitions in the fourth set, but these are irrelevant. {a, b, c, d}, {d, e, a, c}, {d, b, a, c}, {a, a, d, e, c, e} Solution: The first and third sets are the same because they both contain the elements a, b, c and d. The difference is just in the order in which they are listed and it is irrelevant by the axiom of extension. Example 90 Which of the following sets are equal?