Taking out m(m≤n) elements from n different elements and arranging them in a certain order is called Permutation of taking out m elements from n elements. So take the first letter A to indicate how many calculation methods are arranged in this case.

Taking out m(m≤n) elements from n different elements, no matter what order they are combined into a group, is called taking out the combination of m elements from n elements. So use the first letter c to indicate how many ways to calculate the combination.

When it comes to applications, most of them use A when there is order, and C when there is no order. But in some cases, it is more complicated. You should analyze each small step according to the extraction steps, and then choose the calculation method.