设P(x,y)为抛物线上任意一点,则PA^2=(y-a)^2+x^2=y^2-2ay+a^2+4y=(y-(a-2))^2+a^2-(a-2)^2=(y-(a-2))^2+4a-4由于y>=0因此当a-2>=0即a>=2时,当y=a-2,时PA^2有最小值4a-4此时最短距离为根号(4a-4)当a-2...