推 sophialiege :C(n,k)C(n,k-1) = C(n,k)C(n,n-k+1) 04/24 22:36
→ sophialiege :意思是2類東西共取n-1個的取法數 04/24 22:38
推 luke2 :C(2n,n)= sigma(n,1) C(n,n)的平方 往這邊想 04/25 12:03
→ Sfly :let n=1. 1= 3/2 - 2 >< 04/25 12:16
→ Sfly :however, it's ture when 1/2 is cancelled. 04/25 12:22
推 id010406 :這式子是原PO自推的? 不正確 用generating function 04/26 16:39
→ id010406 :左式為(1+x)^n(1+x)^n 的x^(n-1)係數 也就是(1+x)^2n 04/26 16:41
→ id010406 :中x^(n-1)係數為c(2n,n-1) 原右式1/2去掉結果相同 04/26 16:43