LaTeX: Numbering only last line of multi-line equation in \align

I want to number and make a reference to just the last line of a multi-line equation in \align. Here is an example:

\begin{align}
z_1^2 &= \left( 2 e^{i \frac{\pi}{3}} \right) \left( 2 e^{i \frac{\pi}{3}} \right)\\
&= 4 e^{i \frac{2 \pi}{3}}
\end{align}

I only want to number the &= 4 e^{i \frac{2 \pi}{3}} part, rather than both lines of the \align. How do I开发者_Python百科 do this?

From the amsmath documentatiom (amsldoc.pdf):

You can suppress the number on any particular line by putting \notag before the \\;

So your equation should look like this (some bits removed from your equation to make it shorter):

\begin{align}
z_1^2 &= \left( ... \right) \left( ... \right)\notag\\
&= 4 e^{i \frac{2 \pi}{3}}
\end{align}