应是一元二次函数y=ax^2+bx+c顶点坐标(-b/(2a),(-b^2+4ac)/(4a^2))
y=ax^2+bx+c=a(x^2+bx/a+c/a)=a[x^2+bx/a+(b/2a)^2-(b/2a)^2+c/a]=a(x+b/2a)^2-(b^2-4ac)/4a^2
当x=-b/2a时y=-(b^2-4ac)/4a^2