儿童牙齿长歪怎么办:Cn/n+1*Cn-2/n 等于多少?
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需要过程!
C(n,n+1)=C(1,n+1)=n+1
C(n-2,n)=C(2,n)=n(n-1)/2
->C(n,n+1)*C(n-2,n)=(n+1)n(n-1)/2
=3(n+1)n(n-1)/(3*2*1)
=3*C(3,n+1)
C(n,n+1)=C(1,n+1)=n+1
C(n-2,n)=C(2,n)=n(n-1)/2
->C(n,n+1)*C(n-2,n)=(n+1)n(n-1)/2
=3(n+1)n(n-1)/(3*2*1)
=3*C(3,n+1)