Let E1 and E2 be two independent events such that P(E1)=P1 and P(E2)=P2. Describe in words of the events whose probabilities are
1−(1−P1)(1−P2)
P(E1)=P1 and P(E2)=P2
1−(1−P1)(1−P2)=1−P(E1)′P(E2)′=1−P(E′1∩E′2)=1−[1−P(E1∪E2)]=P(E1∪E2)
So, either E1 or E2 or both E1 and E2 occurs.