我该如何正确对齐以下内容？

Question

有什么想法可以让我不用表格就能实现如下图所示的对齐？

Answer 1

使用更友好的输入语法（以代码为代价）：

\documentclass{article}
\usepackage{amsmath,amssymb,xparse}

\ExplSyntaxOn
\NewDocumentEnvironment{derivation}{b}
 {
  \bool_gset_false:N \g_vanderwalt_derivation_therefore_bool
  \seq_set_split:Nnn \l_vanderwalt_derivation_body_seq { \\ } { #1 }
  \begin{alignedat}{2}
  \seq_map_function:NN \l_vanderwalt_derivation_body_seq \vanderwalt_derivation_line:n
  \end{alignedat}
 }{}

\bool_new:N \g_vanderwalt_derivation_therefore_bool
\seq_new:N \l_vanderwalt_derivation_body_seq
\seq_new:N \l_vanderwalt_derivation_line_seq
\tl_new:N \l_vanderwalt_derivation_line_tl

\cs_new_protected:Nn \vanderwalt_derivation_line:n
 {
  \seq_set_split:Nnn \l_vanderwalt_derivation_line_seq { & } { #1 }
  \tl_put_right:Nn \l_vanderwalt_derivation_line_tl { & }
  \bool_if:NTF \g_vanderwalt_derivation_therefore_bool
   { \tl_put_right:Nn \l_vanderwalt_derivation_line_tl { \therefore } }
   { \bool_gset_true:N \g_vanderwalt_derivation_therefore_bool }
  \tl_put_right:Nx \l_vanderwalt_derivation_line_tl
   {
    \seq_item:Nn \l_vanderwalt_derivation_line_seq { 1 }
    & \exp_not:N \qquad &
    \int_compare:nT { \seq_count:N \l_vanderwalt_derivation_line_seq > 1 }
     {
      \exp_not:N \text{(\seq_item:Nn \l_vanderwalt_derivation_line_seq { 2 })}
     }
    \exp_not:N \\
   }
  \tl_use:N \l_vanderwalt_derivation_line_tl
 }

\ExplSyntaxOff

\begin{document}

\begin{equation*}
\begin{derivation}
4x^2 - 12x = -8 \\
x^2 - 3x = -2               & divide all terms by $4$ \\
x^2 - 3x + 2 = 0            & write in standard form \\
(x - 2)(x - 1) = 0          & factorise \\
x-2 = 0 \text{ or } x-1 = 0 & apply the zero factor law \\
x = 2 \text{ or } x = 1
\end{derivation}
\end{equation*}

\end{document}

环境derivation收集其内容并将其拆分为\\。然后处理每一行，并将<formula> & <text>其转换为

& <formula> & \qquad & \text{(<text>)} \\

\therefore从第二行开始添加符号；只有<text>在 之后添加了内容时才会添加括号中的&。

代码在alignedat环境中执行。

这与

\begin{equation*}
\begin{alignedat}{2}
& 4x^2 - 12x = -8 \\
& \therefore x^2 - 3x = -2               &\qquad& \text{divide all terms by $4$} \\
& \therefore x^2 - 3x + 2 = 0            &\qquad& \text{write in standard form} \\
& \therefore (x - 2)(x - 1) = 0          &\qquad& \text{factorise} \\
& \therefore x-2 = 0 \text{ or } x-1 = 0 &\qquad& \text{apply the zero factor law} \\
& \therefore x = 2 \text{ or } x = 1
\end{alignedat}
\end{equation*}

但输入语法要繁琐得多。

Answer 2

您可以通过两个并排的对齐来获得它：

\documentclass{article}
\usepackage{mathtools, amssymb}

\begin{document}

\begin{align*}
 & 4x^2-12x = -8\\
 & \therefore x^2-3x = -2 & & \text{ (divide all terms by 4)}\\
 & \therefore x^2-3x + 2 = 08 & & \text{ (write in standard form)}\\
 & \therefore (x-2)(x-1)=0 & & \text{ (factorise)}\\
 & \therefore x-2 = 0\text{ or } x-1= 0 & & \text{ (apply the zero factor law)}\\
 & \therefore x = 2 \text{ or }x =1
\end{align*}

\end{document} 

Answer 3

这是一个开始吗？

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{align*}
4x^2-12x = -8 & \text{ (Text Text Text)}\\
4x^2-12x = -88 & \text{ (Text Text Text)}\\
4x^2-12x = -8 & \text{ (Text Text Text)}\\
4x^2-12x = -888 & \text{ (Text Text Text)}\\
4x^2-12x = -8 & \text{ (Text Text Text)}\\
4x^2-12x = -88 & \text{ (Text Text Text)}
\end{align*}

\end{document}