Formula P refers to arrangement, and R elements are selected from N elements for arrangement (i.e. sorting). P is an old usage, and now a is often used in textbooks.

Formula C refers to combination, in which R elements are taken from N elements without arrangement (that is, without sorting). Am，n = m *(m- 1)*(m-2)*……*(m-n+ 1)Cm，n=Am，n/n！ = m * (m-1) * (m-2) * ... * (m-n+1)/[n * (n-1) * ... 3 * 2 *1] Welcome to adopt it.