(2011.浙江)如图,在直角坐标系中,抛物线y=ax^2+bx+c与x轴交与点A(﹣1,0)如图,在直角坐标系中,抛物线y=ax^2+bx+c(a不等于0)与x轴交于点A(-1,0),B(3,0)两点,抛物线交y轴与点C（0,3,点D为抛物线顶点直线y=x-1

Question

(2011.浙江)如图,在直角坐标系中,抛物线y=ax^2+bx+c与x轴交与点A(﹣1,0)
如图,在直角坐标系中,抛物线y=ax^2+bx+c(a不等于0)与x轴交于点A(-1,0),B(3,0)两点,抛物线交y轴与点C（0,3,点D为抛物线顶点直线y=x-1交抛物线于点MN两点,过线段MN上一点P做y轴的平行线交抛物线于点Q
问点P坐标为何值时,线段PQ最长,最长为多少?

Answer 1

y=ax^2+bx+c,与x轴交于点A(-1,0),B(3,0)两点a-b+c=0,9a+3b+c=0,2a+b=0,b=-2a,c=-3a交y轴与点C（0,3,c=3,a=-1,b=2,y=ax^2+bx+c=-x^2+2x+3=-(x-1)^2+4,点D为抛物线顶点D(1,4)直线y=x-1交抛物线于点MN两点,x-1=-x^2+2x+...