Complementary events occur when there are just two events, and one event is exactly opposite to the other event. Hence, A∪A¯=setofsamplespaceA∪A¯=setofsamplespace
For an event with probability P(A)P(A), its complement is P(A¯)P(A¯) such that
P(A¯)+P(A)=1P(A¯)+P(A)=1
Example: In an examination, the event of success and the event of failure are complementary events.
P(Success)+P(Failure)=1