\begin{matrix}
a_{11} & a_{12} & a_{13}\\
a_{21} & a_{22} & a_{23}
\end{matrix}

$$\begin{matrix} a_{11} & a_{12} & a_{13}\\ a_{21} & a_{22} & a_{23} \end{matrix}$$

\begin{pmatrix}p_{11}&p_{12}\\p_{21}&p_{22}\end{pmatrix},
\begin{bmatrix}b_{11}&b_{12}\\b_{21}&b_{22}\end{bmatrix},
\begin{Bmatrix}B_{11}&B_{12}\\B_{21}&B_{22}\end{Bmatrix},
\begin{vmatrix}v_{11}&v_{12}\\v_{21}&v_{22}\end{vmatrix},
\begin{Vmatrix}V_{11}&V_{12}\\V_{21}&V_{22}\end{Vmatrix}

$$\begin{pmatrix}p_{11}&p_{12}\\p_{21}&p_{22}\end{pmatrix}, \begin{bmatrix}b_{11}&b_{12}\\b_{21}&b_{22}\end{bmatrix}, \begin{Bmatrix}B_{11}&B_{12}\\B_{21}&B_{22}\end{Bmatrix}, \begin{vmatrix}v_{11}&v_{12}\\v_{21}&v_{22}\end{vmatrix}, \begin{Vmatrix}V_{11}&V_{12}\\V_{21}&V_{22}\end{Vmatrix}$$

\theta(x)=\begin{cases}0&(x<0)\\1&(x\ge 0)\end{cases}

$$\theta(x)=\begin{cases}0&(x<0)\\1&(x\ge 0)\end{cases}$$

\begin{align}\cos 2x &= \cos^2x-\sin^2x\\
                     &= 2\cos^2x-1\\
 \frac{\cos 2x+1}{2} &= \cos^2x
\end{align}

$$\begin{align}\cos 2x &= \cos^2x-\sin^2x\\ &= 2\cos^2x-1\\ \frac{\cos 2x+1}{2} &= \cos^2x \end{align}$$

A\sqrt{B}C,  A\substack{B}C, A\underset{B}C, A\not{B}C

$A\sqrt{B}C, A\substack{B}C, A\underset{B}C, A\not{B}C$

$\color{GreenYellow}{AAA}\overset{a}{\color{blue}x}$