LaTeX Character

    \Rightarrow
    \rightarrow
    \infty
    \equiv
    \approx
    \propto
     \simeq
    \sim
    \neq
     \geq
    \gg
     \ll
    \langle{x}\rangle
     \vec{E}
     \ddot{x}
     \dot{x}
     \partial


Matrix

command:
\left| \begin{array}{ccc}
a & b & c\\
d & e & f \\
g & h & i
\end{array} \right|


Multiline Equation:

command:
\begin{aligned}
x^2+4x-21&=0\\
x^2+4x&=25\\
x^2+4x+4&=21+4\\
(x+2)^2&=25\\
x+2&=\pm 5\\
x&=-2\pm 5
\end{aligned}



     \displaystyle \sum_i^n a_i

\sum_{i=1}^nx_iy_i

     \frac{a}{b}

     \sqrt{z}

 
command: \left. \frac{du}{dx} \right|_{x=0}

Big Size:
  \Huge A^2