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

Division operation

1 divided by a number that is not equal to zero is equal to the reciprocal of this number.

2. Divide two numbers, the same sign is positive and the different sign is negative, and divide by the absolute value. Divide zero by any number that is not equal to zero to get zero.

note:

Zero can't be a divisor and denominator.

Division and multiplication of rational numbers are reciprocal operations.