Prove the Multinomial Theorem: If $n$ is a positive integer, then $\displaystyle{}(x_{1} + x_{2} + \dots + x_{m})^{n} = \sum_{n_{1} + n_{2} + \dots + n_{m} = n}\:\: C(n:n_{1},n_{2},\dots, n_{m})x_{1}^{n_{1}}x_{2}^{n_{2}}\dots x_{m}^{n_{m}},$ where $C(n:n_{1},n_{2},\dots,n_{m}) = \dfrac{n!}{n_{1}!n_{2}!\dots n_{m}!}$ is a multinomial coefficient.
