非会员，不用注册会员也能看！

https://blog.csdn.net/Runcode8/article/details/136274861https://blog.csdn.net/Runcode8/article/details/136274861

---

一、认识C,P,A:

        A.排列        A(x,y)=(x!)/[(x-y)!]=x×(x-1)×...×(x-y+1)

        P.排列        P(x,y)=A(x,y)

        C.组合        C(x,y)=A(x,y)÷(y!)=x×(x-1)×...×(x-y+1)/(y!)=(x!)÷{(y!)×[(x-y)!]}

  例：C(5,2)=(5×4)÷(2×1)=10

  例：