Can I ask for the proof of the Multinomial Theorem? Wikipedia says:
For any positive integer ''m'' and any nonnegative integer ''n'', the multinomial formula tells us how a sum with ''m'' terms expands when raised to an arbitrary power ''n'':
$$(x_1 + x_2 + \cdots + x_m)^n = \sum_{k_1+k_2+\cdots+k_m=n} {n \choose k_1, k_2, \ldots, k_m} \prod_{1\le t\le m}x_{t}^{k_{t}}\,,$$ where $$ {n \choose k_1, k_2, \ldots, k_m} = \frac{n!}{k_1!\, k_2! \cdots k_m!}$$ is a '''multinomial coefficient'''.
But I find the proof they provide on Wikipedia to be insufficiently too brief.