Let (h, k) be any point on either of the tangents that can be drawn to the ellipse.
The equation of the straight line joining (A, k) to (x1, y1) is
If this straight line touch the ellipse, it must be of the form
But this is the condition that the point (h, k) may lie on the locus
This equation is therefore the equation to the required tangents.
It would be found that (1) is equivalent to
