1. The problem statement, all variables and given/known data Attached is the problem 2. Relevant equations 3. The attempt at a solution So I have to show that each side is a subset of the other side Assume x∈ A ∪ (∩Bi) so x∈A or x∈∩Bi case 1 x∈ ∩ Bi so x∈ (B1∩B2∩B3.......∩Bn) which implies x∈B1 and x∈B2 ......... and x∈Bn so x∈B1∪A and x∈B2∪A.......... and x∈Bn∪A so x∈∩(A∪Bi) My teacher told me that this approach doesn't work because the sets can be infinite and told me to use "words". I don't what he meant with that.