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Weighted Combination Sum
## Formula
$$\sum_{r=0}^{n}r{}^nC_r=n2^{n-1}$$
Also:
$$\sum_{r=0}^{n}r(r-1){}^nC_r=n(n-1)2^{n-2}$$
## Usage
- Used in summations involving the index multiplied by a binomial coefficient.
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Weighted Combination Sum
## Formula
$$\sum_{r=0}^{n}r{}^nC_r=n2^{n-1}$$
Also:
$$\sum_{r=0}^{n}r(r-1){}^nC_r=n(n-1)2^{n-2}$$
## Usage
- Used in summations involving the index multiplied by a binomial coefficient.
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