Question

what is the formula for n(AB)

Answer 1

• If n(A) and n(B) denote the number of elements in two finite sets A and B respectively, then for any two overlapping sets A and B, n(A∪B) = n(A) + n(B) - n(A⋂B)
• If A and B are disjoint sets, n(A∪B) = n(A) + n(B)
• If A, B and C are 3 finite sets in U then, n(A∪B∪C)= n(A) +n(B) + n(C) - n(B⋂C) - n (A⋂ B)- n (A⋂C) + n(A⋂B⋂C)