math_equation_test
\[
\frac{1}{n \left( n+k \right)}= \frac{1}{k}\left( \frac{1}{n}-\frac{1}{n+k}\right)
\]
\[
\begin{array}{c}
\text{若}\left \{a_{n}\right \}、\left \{b_{n}\right \}\text{为等差数列}, \\
\text{则}\left \{a_{n}+ b_{n}\right \}\text{为等差数列}
\end{array}
\]
\[
\begin{pmatrix}
a_{11} & a_{12} & a_{13} \\
a_{21} & a_{22} & a_{23} \\
a_{31} & a_{32} & a_{33}
\end{pmatrix}
\]