Prove that if x ∉ B and A ⊆ B, then x ∉ A.

Answer 1

A ⊆ B
=> ∀x x∈A -> x∈B.
=> ∀x ¬ (x∈B) -> ¬ ( x∈A)
=> ∀x x ∉ B -> x ∉ A