The locus of the mid-point of the line segment joining the focus ot a moving point on the parabola y2 = 4ax is another parabola with directrix
x = 0
S=(a,0),P(at2,2at).If(h,k) be mid−point, then 2h=a(1+t2),2k=2at or k=at
∴ 2x =a (1+y2a2 )or y2=2a(x−a2 )or Y2=4AX
∴ Directrix is X = −A or x −a2=−2a4 or x = 0.