What is A \ B (the set difference) for A = {1,2,3} and B = {3,4}?

• A. {3}
• B. {2,3}
• C. {1,3}
• D. {1,2}

Answer: (D)

Set difference asks for the elements that are in the first set but not in the second. Look at A = {1, 2, 3} and remove any elements that also appear in B = {3, 4}. The only element of A that appears in B is 3, so we drop it. What remains are 1 and 2, giving A \ B = {1, 2}. The other possibilities would either keep the 3 (which should be removed) or miss one of the elements 1 or 2, so {1,2} is the correct result.