Simplify (nk)(nk−1).
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Simplify
(nk)(nk−1)
(nk−1)=n!(k−1)!(n−k+1)!|(k−1)!k=k! or (k−1)!=k!k=k∗n!k!(n−k+1)!|(n−k)!(n−k+1)=(n−k+1)!=kn−k+1∗n!k!(n−k)!|n!k!(n−k)!=(nk)=kn−k+1(nk)
\(nk)(nk−1)=(nk)kn−k+1(nk)=n−k+1k∗(nk)(nk)=n−k+1k