\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{align*}
f(x) &= a + b \\
& = c+d
\end{align*}


\begin{eqnarray*}
f(x) &=& a + b \\
& =& c+d
\end{eqnarray*}


\end{document}