The first is multiplication, and the value of k is 1~n- 1. Here is equal to1/2 * 2/3 * 3/4 * ... * n-1/n. The second is the combination number, which is equal to (8*7*6)/(3*2* 1). Generally, 8 and 3 in brackets are replaced by m and n respectively. There is a formula equal to (m * m- 1 * m-2 * m-3 *...* male-female.