# Group and phase velocity

#### Takuza

1. Homework Statement

show that the group velocity and phase velocity are related by

d vgroup = vphase - $\lambda$ ( (d vphase) / (d $\lambda$) )

2. Homework Equations
vphase = $\lambda$f
vgroup = d$\omega$/dk

3. The Attempt at a Solution

dw/dk = $\lambda$f - $\lambda$(d$\lambda$f/d$\lambda$)

dw/dk = $\lambda$f - $\lambda$f

Not really sure where to go with this

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