# Simple multivariable problem w/ ellipse

1. Nov 27, 2011

### MeMoses

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

If u=x^3 + 3xy + y^3, determine du/dx on the ellipse 2x^2+3y^2=1

2. The attempt at a solution

Imagine I just use partial derivative somehow, but I'm not sure what the question is asking by on the ellipse. I have a feeling its just something simple that I'm overlooking. Any help is appreciated. Thanks
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 27, 2011

### HallsofIvy

Staff Emeritus
The chain rule:
$$\frac{du}{dx}= \frac{\partial u}{\partial x}+ \frac{\partial u}{\partial y}\frac{dy}{dx}$$
You get dy/dx from the requirement that $2x^2+ 3y^2= 1$, of course.