$$ f(x)=\left\{
\begin{aligned}
x & = \cos(t) \\
y & = \sin(t) \\
z & = \frac xy
\end{aligned}
\right.
$$
参考:
[1] Latex中的括号用法总结
$$ f(x)=\left\{
\begin{aligned}
x & = \cos(t) \\
y & = \sin(t) \\
z & = \frac xy
\end{aligned}
\right.
$$
参考:
[1] Latex中的括号用法总结