Source TeX:

f(x)=\left\{{\begin{array}{ll} x^2 & \text{si } x < 1 \\[0.8em] \sqrt{x-1} & \text{si } 1 \leq x \leq 4 \\[0.8em] \dfrac{x+1}{6-x} & \text{si } x > 4 \end{array}}\right.