One very useful inequality for the real numbers is x2 ≥ 0, which is valid for any real number x (it is sufficient to consider properties 1.1.1, 1.1.3 and Exercise 1.2 of the previous section). The use of this inequality leads to deducing many other inequalities. In particular, we can use it to find the maximum or minimum of a quadratic function ax2 + 2bx + c. These quadratic functions appear frequently in optimization problems or in inequalities.