# Homework Help: Lagrange on an Ellipse to find Max/Min Distance

1. Nov 29, 2012

### YayMathYay

1. The problem statement, all variables and given/known data

2. Relevant equations

Lagrance Multipliers.

3. The attempt at a solution

This is a pretty dumb question, and I feel a little embarassed asking but..

I know how to do the Lagrange part (I think). I'm assuming you maximize/minimize the distance, $\sqrt{x^{2} + y^{2} + z^{2}}$ subject to the constraints of the ellipse.

The part I'm having trouble with is finding the constraints of the ellipse - could someone help point me in the right direction?

2. Nov 29, 2012

### Ray Vickson

It is a lot easier to maximize or minimize distance-squared; do you see why it is OK to do that? As for the constraints, ask yourself: how can I tell if a given point (x,y,z) is on the ellipse? What are the conditions for that?

3. Nov 29, 2012

### YayMathYay

Ah yes, the thought of it passed my mind, but I didn't think it was viable until you pointed it out - thanks! :)

As for the constraints, I know the equations of the cylinder and the plane that create it.. but when I try to put them together (i.e., $x^{2} + y^{2} = x + y + z$), it doesn't seem to give me the correct equation. I might just be having a brain fart, but I just don't know what I'm missing or doing incorrectly here :(

4. Nov 29, 2012

### YayMathYay

Just gave it an attempt, and my maximization problem became:

Maximize $x^{2} + y^{2} + z^{2}$ subject to $x + y - \sqrt{1 - x^{2}} - \sqrt{1 - y^{2}} = 0$

Is this correct? If so, do I need to substitute out z in terms of x and y? Because I have a feeling that would make things very complicated..

5. Nov 29, 2012

### Ray Vickson

You wrote two equations, not one, so you need two Lagrange multipliers.