# General Equation of the Line of the Major Axis of an Ellipse

1. Apr 19, 2015

### Hugo S

This is a question I have been playing with this week out of curiosity but I keep coming up against brick walls and unenlightening results.

Given the equation of an ellipse, say $$x^{2} - xy + y^2 = 2,$$ I would like to find the equation of the line which passes through the major axis.

I tried solving for x and y and applying a rotation matrix. Doing so, I found that

$$x' = \cos \theta (-\sqrt{8-3y^2} - y) + \sin \theta (-\sqrt{8 - 3x^2} - x)$$
$$y' = \sin \theta (\sqrt{8-3y^2} + y) + \cos \theta (-\sqrt{8 - 3x^2} - x).$$

Setting the second equation to equal zero and solving in terms of θ I ended up with the result: $$\tan \theta = \frac{\sqrt{8-3x^2} + x}{\sqrt{8-3y^2 + y}}.$$

This result doesn't seem to help me as it is ultimately telling me something very trivial that I already know about the relationship between x and y coordinates and the tan function.

I know that my choice of coordinates might not be particularly strategic. Are there any other mistakes I'm making in how I'm approaching this?

Thank you!

2. Apr 19, 2015

### Simon Bridge

You may be able to make better progress by being carefull about describing what you are doing. i.e.:

What are x' and y' and $\theta$?

What is the trivial thing the tan equation ends up telling you?
What is it you hoped it would tell you?

Have you seen.:
http://www.maa.org/external_archive/joma/Volume8/Kalman/General.html [Broken]

Last edited by a moderator: May 7, 2017