Mainly because the environment is not called multiline, it's called multline (without the 'i'). Try this:
[tex]
\begin{multline*}
\tan^n \theta=\tan^{n2}\theta \tan^2 \theta\\ =\tan^{n2}\theta(\sec^2\theta1)=\tan^{n2}\theta\sec^2\theta\tan^{n2}\theta
\end{multline*}
[/tex]
 Warren
