Let f : X → Y . Let A1, A2 be subsets of X and let B1, B2 be subsets of Y . Prove:
a) f(A1 ∪ A2) = f(A1) ∪ f(A2).
b) f⁻¹ (B1 ∩ B2) = f⁻¹ (B1) ∩ f⁻¹ (B2)
c) If B1 ⊆ B2 then f⁻¹ (B1) ⊆ f⁻¹ (B2).