There are at most two opposing events: "at most two female students" and "at least three female students". If A and B are impossible events and A and B are inevitable events, then A and B are mutually opposite events, that is to say, A and B must occur and only one event can occur. It is expressed in mathematical language: if it is, it is said that event A and event B are reciprocal events. Also known as event A and event B are opposite events. That is to say, in each experiment, one and only one of event A and event B must occur. The opposite event of is recorded as. Probability relation of opposing events: P(A)+P(B)= 1. For example, in the dice test, A={ the number of points appearing is even}, b={ the number of points appearing is odd}, A∩B is an impossible event, and A∪B is an inevitable event, so a and b are opposite events.