5.(1)y=4/15(x-5)(x-5/2) (2) Q就是AC与对称轴的交点,Q(15/4,5/6) (3)P1(15/4,-5/12),P2(5/12根号15+15/4,5/12),P3(-5/12根号15+15/4,5/12)6.(1)y=-(x+3)(x-1)(2)设P(x,-(x+3)(x-1)),三角形PCD面积为s,则s=-3/4(x+7/6)2+121/48,存在P(-7/6,143/36),最大面积为121/48