If A, B, C are three sets such that A ∩ B = A ∩ C and A∪ B =A∪ C, then
B = C
A ∩ B = ϕ
A = B
A = C
Let x∈ B ⇒ x ∈ A ∪ B
⇒ x∈ A∪ C⇒x∈ A or x∈ C
If x∈ A , then x∈A∩B=A∩C⇒x∈C
So, x∈ B⇒x∈C
Similarly x∈B⇒x∈ C , Hence B = C .