About alignment of mathematics formulas

Question

How can I implement the alignment like in the following image?

Because the usual \begin{align*} and \end{align*} command can not work, so I hope you can help me!

Thanks! Wait for your elegant codes or solutions...

Answer 1

Actually, you can use align*:

\begin{align*}
   &A(x) \\
={}&B(x) \\
={}&C(x)
\end{align*}

The {} after = is necessary to ensure correct spacing (which is better than what's obtained with eqnarray).

In normal cases one puts the relation symbol after the &, but in this case this would require the cumbersome \mathrel{\phantom{=}} A(x) as in Marco Daniel's answer (which is correct, nonetheless).

One should simply remind that align produces blocks of rl columns, without any space between them (and generous space between blocks). However, it takes special precautions so that a relation symbol after & has the correct spaces, but doesn't before the &. Here the easier solution is to put an empty subformula after the =, which produces the correct space.

Generally the relation symbol goes after the &, which is a rule to be broken only in a few special cases.

Answer 2

One of the golden rules of amsmath is to use the separator before any relation. To suppress the first relation without using the space you can use the commands \mathrel in combination with \phantom:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
&\mathrel{\phantom{=}} A(x) \\
&=B(x) \\
&=C(x)
\end{align*}
\end{document}

Answer 3

Judging from the spacing it looks like a regular old eqnarray (which should never be used anyway).