<s>[INST] Quadratic formula does not work If I put the equation: $5x^2-x-4 =0$  in the quadratic formula, than I get 
$x = 1$ or $x = \frac{-4}{5}$
but the real zeros are: $x = -1$ or $x = \frac{4}{5}$
Can somebody explain me if the quadratic formula fails or me?
 [/INST] $5x^2-x-4$ does indeed have roots $x = 1, -\frac{4}{5}$, so you did it correctly. 
To illustrate:
$$x = \frac{-b \pm \sqrt{b^2-4ac}}{2a}$$
$$=\frac{1 \pm \sqrt{1^2-4*5*(-4)}}{2(5)}$$
$$=\frac{1 \pm \sqrt{81}}{10}$$
$$= \frac{1\pm 9}{10}$$
$$x = 1, -\frac{4}{5}$$
As Peter said if you had meant $5x^2+x-4$ then your roots would have been $x = -1, +\frac{4}{5}$.
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