Align multiple equation to the first line

As @Zarko has already noted in a comment, your objective won't succeed in keeping the typeset material within the document's margins unless they are quite narrow. Even if it does fit -- see the use of \hphantom in the first equation* environment, again as per @Zarko's suggestion -- the resulting look is quite "off balance". I can't say it looks better than what's produced by centering the equation, as usual, in the available space.

Note that I've changed \frac to \tfrac throughout the first four lines of the five-line equation; the frame lines on the sides of the screenshot indicate the edges of the text block.

enter image description here

\documentclass{article}
\usepackage[letterpaper,margin=1in,showframe]{geometry}
\usepackage{amsmath} % for 'equation*' and 'aligned' environments

\begin{document}
\begin{center} centered example line \end{center}

\begin{equation*}
\begin{aligned}
\cos^{-1}{x} &= \sin^{-1}{\tfrac{1}{3}} 
              + \sin^{-1}{\tfrac{7}{9}} \\
  x &= \cos\bigl(\sin^{-1}{\tfrac{1}{3}} 
     + \sin^{-1}{\tfrac{7}{9}}\bigr) \\
\hphantom{\cos\bigl(\sin^{-1}{\tfrac{1}{3}}\bigr) 
       \cos\bigl(\sin^{-1}{\tfrac{7}{9}}\bigr) 
       - \sin\bigl(\sin^{-1}{\tfrac{1}{3}}\bigr) 
         \sin\bigl(\sin^{-1}{\tfrac{7}{9}}\bigr)}
    &= \cos\bigl(\sin^{-1}{\tfrac{1}{3}}\bigr) 
       \cos\bigl(\sin^{-1}{\tfrac{7}{9}}\bigr) 
       - \sin\bigl(\sin^{-1}{\tfrac{1}{3}}\bigr) 
         \sin\bigl(\sin^{-1}{\tfrac{7}{9}}\bigr) \\
    &= \sqrt{1-\bigl(\tfrac{1}{3}\bigr)^2} \,
       \sqrt{1-\bigl(\tfrac{7}{9}\bigr)^2} 
       - \bigl(\tfrac{1}{3}\bigr) 
         \bigl(\tfrac{7}{9}\bigr) \\
    &= 1/3.
\end{aligned}
\end{equation*}

\begin{equation*}
\begin{aligned}
\cos^{-1}{x} &= \sin^{-1}{\tfrac{1}{3}} 
              + \sin^{-1}{\tfrac{7}{9}} \\
  x &= \cos\bigl(\sin^{-1}{\tfrac{1}{3}} 
     + \sin^{-1}{\tfrac{7}{9}}\bigr) \\
    &= \cos\bigl(\sin^{-1}{\tfrac{1}{3}}\bigr) 
       \cos\bigl(\sin^{-1}{\tfrac{7}{9}}\bigr) 
       - \sin\bigl(\sin^{-1}{\tfrac{1}{3}}\bigr) 
         \sin\bigl(\sin^{-1}{\tfrac{7}{9}}\bigr) \\
    &= \sqrt{1-\bigl(\tfrac{1}{3}\bigr)^2} \,
       \sqrt{1-\bigl(\tfrac{7}{9}\bigr)^2} 
       - \bigl(\tfrac{1}{3}\bigr) 
         \bigl(\tfrac{7}{9}\bigr) \\
    &= 1/3.
\end{aligned}
\end{equation*}

\begin{center} centered example line \end{center}
\end{document}

As long as there is no left-hand material in any line but the first, what is requested can be accomplished easily by applying \mathrlap (\usepackage{mathtools}) to the right-hand side of all other lines:

\begin{equation*}
    \text{centered example line}
\end{equation*}

\begin{equation*}
\begin{aligned}
    \cos^{-1}{x} &= \sin^{-1}{\frac{1}{3}} + \sin^{-1}{\frac{7}{9}} \\
    x &= \mathrlap{\cos(\sin^{-1}{\frac{1}{3}} + \sin^{-1}{\frac{7}{9}})} \\
    &= \mathrlap{\cos(\sin^{-1}{\frac{1}{3}}) \cos(\sin^{-1}{\frac{7}{9}}) - \sin(\sin^{-1}{\frac{1}{3}}) \sin(\sin^{-1}{\frac{7}{9}})} \\
    &= \mathrlap{\sqrt{1-(\frac{1}{3})^2} \sqrt{1-(\frac{7}{9})^2} - (\frac{1}{3}) (\frac{7}{9})} \\
    &= \mathrlap{\frac{1}{3}}
\end{aligned}
\end{equation*}

However, as pointed out in a comment by @Zarko, lines with long right-hand sides might well exceed the right margin. And if a document using this convention is submitted for publication to a journal with a narrower text width, it might either be sent back to you for modification or modified by the journal's production staff, opening it to the possibility that errors could be introduced. Your choice.

Tags:

Amsmath

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