The same is true when a < 0 If the line x = a intersects the
parabolas y = x2, y = x2 + 1, y = x2 + 2, y = x2 – 1, y = x2 – 2 at P0, P1, P2, P–1, P–2 etc ,
respectively, then dy
dx  at these points equals 2a This indicates that the tangents to the
curves at these points are parallel