Math & Science ⇒ How to center math items ?

[Cham]

I'm having some difficulties with centering math expressions. Here's a MWE :

Code: Select all

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

\begin{document}

	\begin{align}
		AA,
		&& BBB, \nonumber \\ \\ \nonumber
		CCCCCC,
		&& DDDDD.
	\end{align}

\end{document}

Here's a preview :

pict.jpg (8.84 KiB) Viewed 55552 times

Currently, the items are aligned to their right. How can I have them centered, instead of having them aligned ? I'm sure this is very basic but can't remember how to do it.

[Stefan Kottwitz]

Simply use an array, such as

Code: Select all

\documentclass{article}
\begin{document}
  \begin{equation}
    \begin{array}{c@{\qquad}c}
      AA,      & BBB, \\
      CCCCCC,  & DDDDD.
    \end{array}
  \end{equation}
\end{document}

centering.png (3.57 KiB) Viewed 55539 times

align is better for formulas with alignment at relation symbols, such as equation systems, but here there's no relation, so we don't need align.

Stefan

[Cham]

Thanks Stefan, it works.

[Cham]

I really don't understand the array environment.

Now, I need to present four centered equations, with an aligned arrow between two of them, like this (currently, the arrows aren't aligned) :

Code: Select all

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

\begin{document}

 	\begin{equation*}
	\begin{array}{c@{\qquad}c}
		A = B + C
		\qquad\Rightarrow
		& D = E - F, \\ \\
		G = H
		\qquad\Rightarrow
		& K = P + Q + M.
	\end{array}
	\end{equation*}

\end{document}

Here's a preview :

array.jpg (9.17 KiB) Viewed 55502 times

As you can see, the equations are centered as they should. However, the arrows aren't aligned in the middle.

How can I edit this code to align the arrows in the middle, while keeping the equations centered ?

[Cham]

Hmm, I may have found a solution. Please, could you confirm this is the proper way of doing this, since I really feel unconfortable with the array environment :

Code: Select all

	\begin{equation*}
	\begin{array}{c@{\qquad}c@{\qquad}c}
		A = B + C
		&\Rightarrow
		& D = E - F, \\ \\
		G = H
		&\Rightarrow
		& K = P + Q + M.
	\end{array}
	\end{equation*}

[Johannes_B]

Simply think of array as the maths equivalent of tabular, which to tell the truth it really is.

Code: Select all

 \documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{gather*}
	\begin{array}{c@{\qquad}c@{\qquad}c}
		A = B + C
		&\Rightarrow
		& D = E - F, \\ \\
		G = H
		&\Rightarrow
		& K = P + Q + M.
	\end{array}\\
	\intertext{Now the seperator is combined by a space, an arrow 
	and another space}
	\begin{array}{c@{\qquad\Rightarrow\qquad}c}
		A = B + C & D = E - F, \\
		G = H     & K = P + Q + M.
	\end{array}\\
	\begin{array}{rl@{\qquad\Rightarrow\qquad}rl}
		A &= B + C& D &= E - F, \\ 
		G &= H    & K &= P + Q + M.
	\end{array}
\end{gather*}
\end{document}

[Stefan Kottwitz]

Sure, the proper way is to define a column alignment for each column. The arrows shall be a column, not part of another column, so they can be aligned. As a result, you got three c columns, which is fine.

The second example by Johannes is more like a trick and useful if there's the same arrow in each row.

Stefan

[Cham]

The space is exagerated in the second example, just after the "A", "D", "G" and "K".

But thanks for the reply. I think my problem is solved.