Multinomial Theorem

Multinomial Theorem

## Formula

For non-negative integers $r_1,r_2,\ldots,r_k$ satisfying:

$$r_1+r_2+\cdots+r_k=n$$

we have:

$$(x_1+x_2+\cdots+x_k)^n=\sum\frac{n!}{r_1!r_2!\cdots r_k!}x_1^{r_1}x_2^{r_2}\cdots x_k^{r_k}$$

## Usage

- Used for expansions involving more than two terms.