Source TeX:

f(x)=\left\{{\begin{array}{ll} x^2 & \text{si } x < a_1 \\[0.8em] \sqrt{x-1} & \text{si } a_1 \leq x \leq a_2 \\[0.8em] \dfrac{x+1}{6-x} & \text{si } x > a_2 \end{array}}\right.