The first three items in the formula are listed for people to see the law clearly, and there may not be so many real items. The mistake was that (5-3+ 1) was written at the end, that is, after (5-2) was written at the front, there was nothing at the back, because it was the last item 5-3+ 1.

Calculation method of permutation a and combination c

The calculation method is as follows:

The arrangement a (n, m) = n× (n- 1). (n-m+ 1) = n！ /(n-m)！ (n is subscript and m is superscript, the same below).

Combination C(n, m)=P(n, m)/P(m, m)=n! /m！ (n-m)！

For example, a (4,2) = 4! /2! =4*3= 12。

C(4，2)=4！ /(2! *2! )=4*3/(2* 1)=6。

Basic counting principle in permutation and combination

(1) addition principle: There are n ways to do one thing, and finish it. In the first way, there are m 1 different ways, in the second way, there are m2 different ways, ... and in the n ways, there are mn different ways, so there are n = M 1+M2 to complete it.

(2) The method of the first method belongs to the set A 1, the method of the second method belongs to the set A2, ..., and the method of the n method belongs to the set An, so the method to accomplish this belongs to the set A 1ua2u...uan.

(3) Classification requirements: each method in each category can accomplish this task independently; The specific methods in the two different methods are different from each other (that is, the classification is not heavy); Any method to accomplish this task belongs to a certain category (that is, classification does not leak).