Please help me this is my honework problem
closed as not a real question by Henning Makholm, William, Michael Greinecker♦, Thomas, Aang Sep 25 '12 at 1:53
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An example of a set together with a non-associative binary operator is the integers $\mathbb Z$ with subtraction. For instance, (7 − 2) − 3 = 2, while 7 − (2 − 3) = 8. Because the way you group parentheses gives different results, it follows that subtraction is not associative.