Q:

How to write an array of equations without using eqnarray?

I heard that it is recommended to use align or array. But how should I do so to replace the use of eqnarray?
Thank you very much.

A:

With an eqnarray environment, you might write:
\begin{eqnarray}
a &=& b\\
c &=& d
\end{eqnarray}

With an align environment (be sure to load the amsmath package), you'd write
\begin{align}
a &= b\\
c &= d
\end{align}

The only significant change -- other than the change in the name of the environment, of course -- is that one writes &= (not =&!) when using an align environment in order to bring about alignment on the = symbol.

