Fuente TeX:

 x \leq x^2 + y^2 \implies 0 \leq  x^2 - x + y^2 \implies 0 \leq  x^2 -2 \frac{1}{2} x + y^2  \\ \implies (\frac{1}{2})^2 \leq   x^2 -2 \frac{1}{2} x + (\frac{1}{2})^2 + y^2 \implies \frac{1}{4} \leq (x-\frac{1}{2})^2 +y^2