配方得的,f(x)=ax^2+bx+c =a(x^2+b/a*x)+c =a[x^2+b/a*x+(b^2/4a^2)]-b^2/4a^2+c =a(x+b/2a)^2+(4ac-b^2)/4a^2 当x=-b/2a时,有最值(4ac-b^2)/4a^2即y=a(x+b/2a)^2+(4ac-b^2)/4a^2 祝你学习天天向上,加油!...