Multinomial Theorem

Theorem

$(x_1+x_2+\cdots +x_l{)}^n={\sum }_{k_1,k_2,\dots ,k_n}(\genfrac{}{}{0ex}{}{n}{k_1,k_2,\dots ,k_n})x_1^{k_1}{x}_2^{k_2}\dots x_l^{k_l}$ Where $k_1+k_2+\cdots +k_n=n$