y=ax^2+bx+c(a≠0）,过原点,X=0,Y=0代入,即得C=0,顶点,即最值（1,3）,-b/2a=1,(4ac-b^2)/4a=3,因c=0,可解出a=-3,b=6,(b=0舍）,即F（X）=-3x^2+6x