Finding the points on an ellipse where the slope of the tangent line equals 1

[delriofi]

If there is an an ellipse x^2/9 + y^2/16 = 1, and the slope of the tangent is dx/dy = -16x/9y, how do you find what points at which the slope of the tangent is 1? I have no idea how to answer this and I've been trying for like an hour. Can anyone help me?

[micromass]

Hi delriofi! :smile:

If there is an an ellipse x^2/9 + y^2/16 = 1, and the slope of the tangent is dx/dy = -16x/9y, how do you find what points at which the slope of the tangent is 1? I have no idea how to answer this and I've been trying for like an hour. Can anyone help me?

Don't you mean dy/dx=-16x/9y ?
Well, since you know the slope is 1, you know that 1=-16x/9y, thus 9y=-16x. Substitute that back into the equation for the ellipse and solve for x (or y).

[delriofi]

Ok so I did x^2/9 + (-9y/16)^2/16 = 1 and I get that x^2/9 - x^2/9 = 1 but that doesn't make sense does it?

[Hurkyl]

This is a duplicate of this post:

http://www.physicsforums.com/showthread.php?p=3330285#post3330285


I'm closing this one.