Quadratic formula not working? Suppose I want to find the $ n $ for which 
$$(n)(n+1)/2 = 10 \Longrightarrow n^2 + n - 20 = 0$$
Clearly a solution is $4$. But, suppose we wanted to find that solution by use of quadratic formula. We get
$$4 = \frac {-1 \pm \sqrt{1-80}}{2} $$
But that's clearly not true. What am I doing wrong?

$$Ax^2+Bx+C=0\implies x=\frac{-B\pm\sqrt{B^2-4AC}}{2A}$$
$$\implies n=\frac{-1\pm\sqrt{1^2-4\cdot1\cdot(-20)}}{2\cdot1}$$
