When $a \ne 0$, there are two solutions to \(ax^2 + bx + c = 0\) and they are
$$x = {-b \pm \sqrt{b^2-4ac} \over 2a}.$$
잘 되는군요.
다른 것들
$f\left( x \right) =f\left( 0 \right) -\sum _{ n=1 }^{ \infty }{ \frac { \left( -x \right) ^{ n } }{ n! }\cdot \frac { d^{ n } }{ dx^{ n } }f\left( x \right) } $
$$\sum _{ k=1 }^{ 2n }{ \left( -1 \right) ^{ k } }f\left( k \right) =\sum _{ k=1 }^{ n }{ \{f\left( 2k \right) -f\left( 2k-1 \right) \} } $$
\(\frac { i }{ 2 }\ln\frac { x+i }{ x-i }+\frac { \pi }{ 2 }=\arctan x \)
