LaTeX代码可以直接论坛使用了

\begin{equation}
\begin{aligned}
E\left ( \varepsilon _{n}\right )
&=E\left ( v_{n}t_{n} \right )+E\left ( X_{n}^{\left ( v_{n} \right )} \right )\\
&=E\left ( v_{n}t_{n} \right )+E\left [ E\left ( X_{n}^{\left ( v_{n} \right )}\mid v_{n} \right ) \right ]\\
&=\frac{R}{1-R}\cdot \frac{t_{1}}{a^{n-1}}+\frac{1}{1-R} \int_{0}^{t_{n}}tdF\left ( a^{n-1}t \right )\\
&=\frac{1}{\left ( 1-R \right )a^{n-1}}\int_{0}^{t_{1}}\left ( 1-F\left ( t \right ) \right )dt
\end{aligned}
\end{equation}

$\sigma$

\alpha

\begin{eqnarray}
e^x &\approx& 1+x+x^2/2! + \\
   && {}+x^3/3! + x^4/4! + \\
   && + x^5/5!
\end{eqnarray}

try

\begin{align}
\text{e}^{i\pi}=-1
\end{align}

try
$\alpha$

$$\text{e}^{i\pi}=-1$$

$\text{e}^{i\pi}=-1$

failure...

Very good! 要等会儿才能看到啊