# Staggered levels of alignment ; align to parts of previous line that weren't aligned with the lines before it

How can I align the maths equation as I've aligned it in the source?

\begin{align}
&\sin(A+B)=\sin A\cos B + \sin B\cos A \\
&\sin(A-B)=\sin A\cos B - \sin B\cos A
%
\intertext{It's possible to eliminate the $\sin B\cos A$ terms by summing the two
equations, it will then be possible to rearrange for
$\sin A\cos B$:}
%
(2)+(3): &\sin(A+B) + \sin(A-B)&=2\sin A\cos B \nonumber\\
2\sin A\cos B &=\sin(A+B) + \sin(A-B) \nonumber
\end{align}


Here's what it currently looks like, with annotations as to how I'd like it to look: I don't really care about it being centred, I think it just looks uncomfortably left-aligned as it currently is.

I plan to add more lines of display maths below the current last one, which I'd also like to align with the equal sign of that first unnumbered line.

## 2 Answers

Nested aligned, with less code and the correct numbers without guessing at them.

However, the alignment is not at all necessary, particularly in the last line. I add an alternative solution. The reverse equality doesn't seem useful, though.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\section{How you want it}

\begin{align}
&\sin(A+B)=\sin A\cos B + \sin B\cos A \label{sum}\\
&\sin(A-B)=\sin A\cos B - \sin B\cos A \label{diff}
\intertext{It's possible to eliminate the $\sin B\cos A$ terms by summing the two
equations, it will then be possible to rearrange for
$\sin A\cos B$:}
\eqref{sum}+\eqref{diff}{:}\qquad
&\begin{aligned}[t]
\sin(A+B) + \sin(A-B)&=2\sin A\cos B \\
2\sin A\cos B &=\sin(A+B) + \sin(A-B)
\end{aligned}
\notag
\end{align}

\section{How I would do it}

\begin{gather}
\sin(A+B)=\sin A\cos B + \sin B\cos A \label{sum+}\\
\sin(A-B)=\sin A\cos B - \sin B\cos A \label{diff+}
\end{gather}
It's possible to eliminate the $\sin B\cos A$ terms by summing the two
equations, it will then be possible to rearrange for $\sin A\cos B$:
\begin{equation*}
\makebox[0pt][r]{$[\eqref{sum+}+\eqref{diff+}]$\qquad}
\begin{gathered}
\sin(A+B) + \sin(A-B)=2\sin A\cos B \\
2\sin A\cos B =\sin(A+B) + \sin(A-B)
\end{gathered}
\end{equation*}

\end{document} Use a nested aligned:

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

\begin{document}

\begin{align}
&\sin(A+B)=\sin A\cos B + \sin B\cos A \\
&\sin(A-B)=\sin A\cos B - \sin B\cos A
%
\intertext{It's possible to eliminate the $\sin B\cos A$ terms by summing the two
equations, it will then be possible to rearrange for
$\sin A\cos B$:}
%
(2)+(3): \qquad\notag&\smash{\begin{aligned}[t]\sin(A+B) + \sin(A-B)&=2\sin A\cos B \\
2\sin A\cos B &=\sin(A+B) + \sin(A-B)
\end{aligned}}\\[-0.05ex]%
\end{align}

\end{document} • In \begin{aligned}[t] what's the [t] for? – theonlygusti Sep 20 '17 at 16:11
• the [t] says to align the baseline of the top line with the baseline of what precedes the &. – barbara beeton Sep 20 '17 at 16:21
• How can I add a numbered equation at the very end of that aligned environment? – theonlygusti Sep 20 '17 at 16:23
• By default, it will be placed on the line of the last but one equation. I don't know if there's a simple solution. – Bernard Sep 20 '17 at 16:33
• It's because it's defined in the amsmath documentation, but it's the same. Also, it's shorter to type. – Bernard Sep 20 '17 at 16:49