Question

Prove that A∩U=A

Answer 1

I'm assuming by U, you mean the universal set?
To prove equality between sets, show that they are subsets of each other :)
To prove that a set is a subset of another, show that if you take an element of the first set, it is certainly in the second set...
Can you do it from here? :)

Answer 2

its the venn diagram of A intersecton U