连乘\prod_{i=1}^n
\[\prod_{i=1}^n
\]
分数\frac{a}{b}
\[\frac{a}{b}
\]
组合数\tbinom{n}{r}=\tbinom{n}{n-r}=C^n_r=C^n_{n-r}
\[\tbinom{n}{r} = \tbinom{n}{n-r} = C^n_r = C^n_{n-r}
\]
\(\begin{align*}x^n = \sum\limits_{k=1}^n\left\{\begin{matrix}n\\k\end{matrix}\right\}x^\underline k\end{align*}\)