一般能够用到的环境是
\begin{equation}
\begin{aligned}
...
\end{aligned}
\end{equation}
然而，这种环境用&只能够保证左对齐或者右对齐，用\begin{gather*}环境能够保证居中对齐
如下
\begin{gather*}
r_u = (1+k_1r^2_d+k_2r^4_d)r_d \\
r_u = \frac{tan(r_d\omega)}{2tan{\frac{\omega}{2}}}\\
r_u = \frac{r_d}{1+kr^2_d},
\end{gather*}
没毛病