Let 0<P(A)<1,0<P(B)<1 and P(A∪B)=P(A)+P(B)−P(A)P(B), then
P(A∣B)=0
P(B∣A)=0
PA′∩B′=PA′PB′
P(A∣B)+P(B∣A)=1
We know that
P(A∪B)=P(A)+P(B)−P(A∩B)⇒P(A)+P(B)−P(A)P(B)=P(A)+P(B)−P(A∩B)⇒P(A)⋅P(B)=P(A∩B)⇒A and B are independent.