Math & Science ⇒ Aligning items in a broken equation

[Cham]

I need to align some terms in an equation which is splited on two lines, like the compilable example below.

Currently, I'm using the \phantom command to align the last terms (on the second line) to the minus sign of the first line. This is a "hack". What is the proper way to do this ?

Code: Select all

\documentclass[12pt,letterpaper]{article}
\usepackage[T1]{fontenc}
\usepackage{amsmath}

\begin{document}
\noindent
Bla bla bla bla :
	\begin{align*}
		A &= B + C - D \\ \\
		%phantom
		&\phantom{= B + C \;}
		%phantom
		 + D - E \\ \\
		&= F + G - H.
	\end{align*}
\end{document}

Here's a preview :

align.jpg (9.4 KiB) Viewed 4664 times

[Stefan Kottwitz]

Hi Cham,

good MWE! The proper way is to use alignat, such as:

Code: Select all

\begin{alignat*}{2}
  A &= B + C &&- D \\
    &        &&+ D - E \\
    &= F + G &&- H.
\end{alignat*}

Stefan

[Cham]

Ahaa !

Thanks for the fast answer, Stefan !

This should solve my problem.

[Cham]

Hmm, no, there's a small problem with this.

The third line shouldn't have its items aligned with the lines above it. Only its "=" sign should be aligned with the equal sign of the first line.

[Stefan Kottwitz]

Make another hack. Just joking.

Code: Select all

\documentclass[12pt,letterpaper]{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools}
\begin{document}
\begin{alignat*}{2}
  A &= B + C &&- D \\
    &        &&+ D - E \\
    &= \mathrlap{F + G - H}.
\end{alignat*}
\end{document}

Sometimes, tweaking is necessary. However, problems with the commonly used environments may also be a hint that the desired formatting might be a bit unusual.

Stefan

[Cham]

The \mathrlap command appears to be the solution. This one was really unexpected !

[Cham]

Sometimes, tweaking is necessary. However, problems with the commonly used environments may also be a hint that the desired formatting might be a bit unusual.

Well, the actual equation is too long to fit on a single line. This is why I splited the first equation on two lines. However, there's a second medium-long equation on the third line and I need it to be aligned with the equal sign of the first equation, like the example above.

So if this formatting is a bit unusual, I wonder how should I format both equations.

[Stefan Kottwitz]

A proper way would be using blocks, also provided by amsmath.

Code: Select all

\begin{align*}
  A &= B + C
    \begin{aligned}[t]
      &- D \\
      &+ D - E
    \end{aligned} \\
    &= F + G - H.
\end{align*}

Stefan

[Cham]

I prefer the previous solution. With the last solution, I'm getting an extra space around an aligned "+", because of the invisible \right. delimiter. I don't understand why it's doing this.