<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] Given $5x^2 −x−4=0 $ you must use, $a=5, b=-1, c=-4$ in the formula$$\begin{align}x & =\frac{-b\pm\sqrt{b^2-4ac}}{2a} \\[2ex] & = \frac{-(-1)\pm\sqrt{(-1)^2-4(5)(-4)}}{2(5)} \\[2ex] & = \frac{1\pm\sqrt{1+80}}{10}\\[2ex] & = \frac{1\pm 9}{10}\\ \therefore x\in \{-\tfrac {4}{5}, 1\}\end{align}$$
</s>