# Thread: Finding the equation of a derivative line wihtout knowing coordinates

1. ## Finding the equation of a derivative line wihtout knowing coordinates

I haven't figured out how to type equations on here... I'll have to withhold the numbers till I figure it out.

Anyway, you know the equation of the ellipse and the coordinates of point Q. You have to find the equation of the line.

Finding the derivative is easy enough, but I'm not sure where to go from there....

2. Originally Posted by iflyboats

I haven't figured out how to type equations on here... I'll have to withhold the numbers till I figure it out.

Anyway, you know the equation of the ellipse and the coordinates of point Q. You have to find the equation of the line.

Finding the derivative is easy enough, but I'm not sure where to go from there....
If I understand your question correctly you are looking for the equation of the tangent line which passes through Q(q, 0) and has P as the tangent point.

If so:

1. All lines passing through Q have the equation: y = m (x -q)

2. Determine the Points of intersection between the line and the ellipse:

x^2/a^2 + (m(x-q))^2/b^2 = 1

DSolve for x.

3. To get a tangent point the discriminant must be zero. And this is the additional condition to find the tangent line.