解:x=2pt2 y=2pt
可得y^2=4x
设oa的斜率为k,A(a^2,2a)
k(OA)=2/a
OA⊥OB
k(OB)=-a/2
OB:y=-ax/2,x=-2y/a
y^2=4x=4*(-2y/a)
yB=-8/a,xB=16/a^2
P(x,y)
xA+xB=2x,yA+yB=2y
a^2+16/a^2=2x.(1)
2a-8/a=2y
a-4/a=y
(a-4/a)^2=y^2
a^2+16/a^2-8=y^2.(2)
(1)代入(2),得
2x-8=y^2
线段AB的中点P的轨迹方程是抛物线:2(x-4)=y^2