How many elements have P (A), if A= ϕ?

If A is any set, prove that: A ϕ A=ϕ.

Prove that: A B, B C and C A A = C.

What universal set (s) would you propose for each of the following:

(i) The set of right triangles.

(ii) The set of isosceles triangles

If A and B are two sets such that A ⊂ B, then Find:

A ∩ B