Question: a and b are disjoint sets?
Answer: Two sets A and B are said to be disjoint, if they have no element in common. Thus, A = {1, 2, 3} and B = {5, 7, 9} are disjoint sets; but the sets C = {3, 5, 7} and D = {7, 9, 11} are not disjoint; for, 7 is the common element of A and B. Two sets A and B are said to be disjoint, if A ∩ B = ϕ.