【论坛公告】新功能：现在可以使用数学公式啦！
教程：http://www.tecbbs.com/forum.php?mod=viewthread&tid=6978
记得勾选“禁用表情”
它这个需要等待你加载好JS后才会解析，然后变身。
$$\sum_{i=1}^n a_i=0$$
$$f(x)=x^{x^x}$$
$$\frac{1}{4}$$
表示开平方:$$\sqrt{x^4}$$
表示开 n 次方: $$\sqrt{(a+b)^4}$$
$$\vec{a} \cdot \vec{b}=0$$
$$\prod_{i=0}^n \frac{1}{i^2}$$
$$f(x\_1,x\_x,\ldots,x\_n) = x\_1^2 + x\_2^2 + \cdots + x\_n^2$$
$$\$$
$$
\begin{eqnarray*}
\cos 2\theta & = & \cos^2 \theta - \sin^2 \theta \\\\
& = & 2 \cos^2 \theta - 1.
\end{eqnarray*}
$$
The characteristic polynomial $\chi(\lambda)$ of the $3 \times 3$~matrix
$$
\left( \begin{array}{ccc}
a & b & c \\\\
d & e & f \\\\
g & h & i \end{array} \right)
$$
is given by the formula
$$
\chi(\lambda) = \left| \begin{array}{ccc}
\lambda - a & -b & -c \\\\
-d & \lambda - e & -f \\\\
-g & -h & \lambda - i \end{array} \right|.
$$
$\frac{du}{dt} $ and $\frac{d^2 u}{dx^2}$
$$\frac{\partial u}{\partial t}
= h^2 \left( \frac{\partial^2 u}{\partial x^2}
+ \frac{\partial^2 u}{\partial y^2}
+ \frac{\partial^2 u}{\partial z^2}\right)$$
$$\lim_{x \to +\infty}, \inf_{x > s} , \sup_K$$
$$ \lim_{x \to 0} \frac{3x^2 +7x^3}{x^2 +5x^4} = 3.$$
$$\sum_{i=1}^{2n}.$$
$$\sum_{k=1}^n k^2 = \frac{1}{2} n (n+1).$$
$$\int_a^b f(x)\,dx.$$
$$ \int_0^{+\infty} x^n e^{_x} \,dx = n!.$$
$$ \int \cos \theta \,d\theta = \sin \theta.$$
$$ \int_{x^2 + y^2 \leq R^2} f(x,y)\,dx\,dy= \int_{\theta=0}^{2\pi} \int_{r=0}^R
f(r\cos\theta,r\sin\theta) r\,dr\,d\theta.$$
$$ \int_0^R \frac{2x\,dx}{1+x^2} = \log(1+R^2).$$
技术宅的结界管理组 9/2/2017
卧槽这JS好慢啊 这玩意儿好像还会影响默认字体
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