[Tugindia] Problems with split environment

[A.S. Diwakar]

Thanks all for your kind reply,

Just for an example I've given the codes. But, actual code I'd given is 
like the one which follows

\documentclass[12pt]{book}
\usepackage{amsmath}
\begin{document}

\begin{equation*}
\begin{split}
x+y-z&=1\\
x-y+z&=1\\
x+y+z&=1
\end{split}
\end{equation*}

\end{document}

The above one gives me some error and not compiled. Please give me some 
ideas about possibility of changing fonts like helvetica, times as in 
others programs.

Balaji