\[\begin{aligned} ax^2+bx+c&=0\\ x^2+\frac{b}{a}x&=-\frac{c}{a}\\ x^2+\frac{b}{a}x+\left(\frac{b}{2a}\right)^2&=-\frac{c}{a}+\left(\frac{b}{2a}\right)^2\\ \left(x+\frac{b}{2a}\right)^2&=-\frac{c}{a}+\frac{b^2}{4a^2}\\ x+\frac{b}{2a}&=\frac{\pm\sqrt{b^2-4ac}}{2a}\\ x&=\dfrac{-b\pm\sqrt{b^2-4ac}}{2a}.\\ \end{aligned}\\\] 标签：right,frac,默写,一元二次方程,2a,求根,left From： https://www.cnblogs.com/caijianhong/p/17006361.html