3
C10=
10!
-------- =
3!(10-3)!
10x9x8x7x6x5x4x3x2x1
-----------------------=120
3x2x1x7x6x5x4x3x2x1
3
A10 = 10x9x8=720
顺便说一下,那不叫C上3下10,是10取3的组合数.
3
A10 应该叫10取3排列数.