(a) With conditional probability, ℙ(A|B), the axioms of probability hold for the event on the left side of the bar. A useful consequence is applying the complement rule to conditional probability. We have that ℙ(A|B) = 1 − ℙ(A|B).
Prove this by showing that ℙ(A|B) + ℙ(A|B) = 1 (Hint: just use the definition of conditional probability, a proof should be very short).
(b) If two events A and B are independent, then we know ℙ(A ∩ B) = ℙ(A)ℙ(B). A fact is that if A and B are independent, they so are all combinations of A, B, ... etc.
Show that if events A and B are independent, then ℙ(A ∩ B) = ℙ(A)ℙ(B), and thus A and B are independent. (Hint: ℙ(A ∩ B) = 1 − ℙ(A ∢ B). Then use addition rule and simplify.)