# Homework Help: Group Velocity

1. Aug 21, 2009

### mrausum

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

Show that:

$$\frac{dc}{d\omega}=\frac{1}{k}\left(1-\frac{c}{v_{g}}\right)$$

3. The attempt at a solution

$$\frac{dc}{d\omega}=\frac{dc}{d\omega}\frac{d\omega}{d\omega}=\frac{dc}{d\omega}\frac{dck}{d\omega}=\frac{dc}{d\omega}k+\frac{dk}{d\omega}c=\frac{dc}{d\omega}k+\frac{c}{v_{g}}=\left\frac{1}{1-k}\right\frac{c}{v_{g}}$$

...Yeah, I have no clue. $$\omega$$, $$\lambda$$ and c are all functions of each other, and i'm confused :(.

Last edited: Aug 21, 2009
2. Aug 21, 2009

### mrausum

Nevermind, solved it. I was really close, I just made a stupid mistake with my algebra.