对齐环境中的多重对齐

Question

我的 MWE：

\documentclass{report}
\usepackage{amsmath}

\begin{document}

\begin{align}
&u = \frac{1}{\theta_{2,3}} \left [ -z_1 - \left ( c_2 + \frac{\epsilon}{2} \right )z_2 - \theta_{2,1}\sin(x_1) - \hat{\theta}_{2,2}x_2 + \dot{\alpha}_1 \right ] \\ 
&\dot{\xi}_{2,2} = - \frac{\partial \beta_2}{\partial x_2}\left [ \theta_{2,1}\sin(x_1)+\left ( \xi_{2,2}  + \beta_2(x_2) \right ) x_2 + \theta_{2,3}u \right ]  \\
&\hat{\theta}_{2,2} = \xi_{2,2} + \beta_2(x_2) \\
&\beta_2 =  \frac{\gamma}{2}x_2^2 
\end{align}

\end{document}

结果：

我想要的是：

也就是说，除了左对齐之外，我想将等式（2）至（4）的等号对齐。

Answer 1

如果想要two最后三个方程的对齐点，可以使用amsmath` 中定义的alignat环境与\mathrlap命令相结合mathtools (which loads：

\documentclass{report}
\usepackage{mathtools}

\begin{document}

\begin{alignat}{2}
&u \mathrlap{{}=\frac{1}{\theta_{2,3}} \left [ -z_1 - \left ( c_2 + \frac{\epsilon}{2} \right )z_2 - \theta_{2,1}\sin(x_1) - \hat{\theta}_{2,2}x_2 + \dot{\alpha}_1 \right ]} \\
&\dot{\xi}_{2,2} &&= - \frac{\partial \beta_2}{\partial x_2}\left [ \theta_{2,1}\sin(x_1)+\left ( \xi_{2,2}  + \beta_2(x_2) \right ) x_2 + \theta_{2,3}u \right ]  \\
 & \hat{\theta}_{2,2} & &= \xi_{2,2} + \beta_2(x_2) \\
&\beta_2 &&= \frac{\gamma}{2}x_2^2
\end{alignat}

\end{document}