TeX source:

f(x)=\left\{\begin{array}{lll} \text { formula } 1 & \text { if } x & \text { is in domain } 1 \\ \text { formula } 2 & \text { if } x & \text { is in domain } 2 \\ \text { formula } 3 & \text { if } x & \text { is in domain } 3 \end{array}\right.