TeX source:

\mathrm{f}(\mathrm{x})=\left\{\begin{array}{ll}\mathrm{Ax}+\mathrm{B} & \text { if } \mathrm{x} < 1 \\ \mathrm{x}^{2}+\mathrm{x} & \text { if } \mathrm{x} \geq 1\end{array}\right.