公式 $f(n) = \sum_{k=0}^{n-1}f(k)f(n-1-k)=\frac{C(2n,n)}{n+1}$ ,测试

$\begin{alignedat}{2} 10&x+ &3&y = 2 \\ 3&x+&13&y = 4 \end{alignedat}$ $\def\arraystretch{1.5} \begin{array}{c:c:c} a & b & c \\ \hline d & e & f \\ \hdashline g & h & i \end{array}$

test change from copy of local machine.

mathe 来测试一下: $e^{i\pi} +1 =0$

A brief introduction about various uses of var, let, const .