吳聰敏老師的討論區

[tmwu]

Try:

\ctxfon
\[
f(x) = \left\{ {{\begin{array}{*{20}c}
{3x - 1} & {;x < 1} \\
{\mbox{無定義}} & {;x = 1} \\
{2x} & {;x > 1} \\
\end{array} }} \right.
\]
\ctxfoff