F(1,0)
设P(x1,y1),Q(x1/2,y1/2)M(x,y)
y1^2=4x1
x=(x1/2+1)/2 (1)
y=y1/4 (2)
得x1=4x-2
y1=4y
代入y1^2=4x1
得y^2=x-1/2