Question

C（8,n）＝70 这个怎么算n,排列组合问题

Answer 1

建议还是用枚举法,直接算过于复杂了：

c(8 1) = 8/1 = 8

c(8 2) = 8*7/(2*1) = 28

c(8 3) = 8*7*6/(3*2*1) = 56

c(8 4) = 8*7*6*5/(4*3*2*1) = 70

c(8 5) = c(8 3) = 56

...

因此 n=4