一般能够用到的环境是
egin{equation}
egin{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egin{gather*}环境能够保证居中对齐
如下
egin{gather*}
r_u = (1+k_1r^2_d+k_2r^4_d)r_d \
r_u = frac{tan(r_domega)}{2tan{frac{omega}{2}}}\
r_u = frac{r_d}{1+kr^2_d},
end{gather*}
没毛病