Mathematics · supplementary
Sum of combinations with weights \(r(r-1)\) for JEE
Permutations and combinations
Formula
\(\displaystyle \sum_{r=0}^{n} r(r-1)\,{}^nC_r=n(n-1)2^{n-2}\)
Variables: \(n\) is a non-negative integer.
Conditions: Valid for \(n\ge 2\).
Higher weighted binomial sums.