Hockey-Stick Identity

Rarely Tested

Hockey-Stick Identity

## Formula

$$\sum_{r=k}^{n}{}^rC_k={}^{n+1}C_{k+1}$$

More generally:

$$\sum_{r=m}^{n}{}^rC_{r-m}={}^{n+1}C_{n-m+1}$$

## Usage

- Used for evaluating sums of binomial coefficients arranged along diagonals of Pascal's triangle.

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