At most, it means ≤ For example, there are three scenarios, namely A, B and C, and at most one scenario occurs. It means that in these three possibilities, either none of the scenes happened or only one scene happened, but never two scenes happened at the same time, let alone other situations where more than two scenes happened. Therefore, the mathematical expression of this sentence is a summary of the following situations:

None of the scenes happened, that is, "non-A, non-B and non-C".

There is only one situation: A occurs without B and C, B occurs without A and C, and C occurs without A and B. ..

That is, the origin of the long series of formulas added in the first line of the diagram.

introduce

Probability theory is a branch of mathematics that studies the quantitative laws of random phenomena. Random phenomenon is relative to decisive phenomenon, and the inevitable occurrence of a certain result under certain conditions is called decisive phenomenon.

The probability of an event is a measure of the possibility of an event. Although the occurrence of an event in random trials is accidental, those random trials that can be repeated in large numbers under the same conditions often show obvious quantitative laws.