$$ a_{1} \qquad x^{-2_{a}} \qquad a^{3}_{ij}$$
$$ \sqrt[3]{x^{2}-\sqrt{y_{1}}} $$
$$ \overline{m + n}\qquad \underline{m-n}$$
$$\overbrace{a+b+...+n}\qquad \underbrace{a+b+.....+n}$$
$$\vec{a} \qquad \overrightarrow{AB} $$
$$ \forall x \in \mathbf{R}: \qquad x\in\mathbb{R} \qquad x^{2} \geq 0$$
$$ \geq \qquad \leq \qquad= \qquad \neq$$
$$\lim_{n \to \infty}\sum_{k=1}^{n} \frac{1}{k^2}=\frac{\pi^2}{6}$$
$$\lambda,\xi,\pi,\mu,\Phi,\Omega$$
$$y’=2x\qquad y''=2$$
$$\lim_{x \rightarrow 0}\frac{\sin x}{x}=1$$
$${n \choose k} \qquad {x \atop y+2}$$
$$\sum_{i=1}^{n} \qquad \int_{0}^{\frac{\pi}{2}}$$
$$1 + \left( \frac{1}{ 1-x^{2} }\right) ^3$$
$$\big(\Big(\bigg(\Bigg(\quad\big\}\Big\}\bigg\}\Bigg\} \qquad \big\|\Big\|\bigg\|\Bigg\|$$
$$\mathbf{X} =\left( \begin{array}{ccc}x_{11} & x_{12} & \ldots \\x_{21} & x_{22} & \ldots \\\vdots & \vdots & \ddots \end{array} \right)$$
$$y = \left\{ \begin{array}{ll}a & \textrm{if $d>c $} \\ b+x & \textrm{in the morning}\\l & \textrm{all day long}\end{array} \right. $$
$$\left(\begin{array}{c|c}1 & 2 \\\hline3 & 4\end{array}\right)$$
$$\left|\begin{array}{} a_{11} & b_{12} \\ b_{21} & a_{22} \end{array}\right|$$
下面是常用的一些数学符号：