1 and c indicate the combination mode. For example, there are three ways for three people, A, B and C, to take out two people to participate in the activity, namely A, B, C and B, not sequentially, but only in combination.

2.p stands for arrangement method, which means to arrange some objects in order. What is the general method?

For example, c (5,2) = (5 * 4)/(2 *1) =10, and c (7,3) = 7 * 6 * 5/3 * 2 *1= 35.

P(5，3)=5*4*3=60，P(6，2)=6*5=30

Extended data:

Some probabilistic addition rules:

Theorem: Let A and B be mutually incompatible events (AB=φ), then:

P(A∪B)=P(A)+P(B)

Inference 1: If A 1, A2, …, An are incompatible with each other, then: P (A1+A2+...+An) = P (A1)+P (A2)+…+P (An).

Inference 2: Let A 1, A2, ..., A form a complete event group, then: P (A1+A2+...+An) =1.

Inference 3:?

Contrary to event a.

Inference 4: if b contains a, then P (B-A) = P (B)-P (A).

Inference 5 (generalized addition formula):

For any two events a and b, there is P (A ∪ B) = P (A)+P (B)-P (AB).

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