x^2=4ay
Focus (0,a)
y = x^2 / 4a
y' = x / 2a
At some point (2ap,ap^2)
y' = 2ap / 2a
= p
Therefore, gradient of normal = -1/p
Eqn of normal: y -ap^2=-1/p(x-2ap)
py-ap^3=-x+2ap
x+py=ap^3+2ap
Through (0,a) (focus)
ap = ap^3 + 2ap
0 = ap^3 + ap
0 = ap (p^2+1)
Therefore, p = 0 only
That means...