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**1. Homework Statement**

show that the group velocity and phase velocity are related by

d v

_{group}= v

_{phase}- [itex]\lambda[/itex] ( (d v

_{phase}) / (d [itex]\lambda[/itex]) )

**2. Homework Equations**

v

_{phase}= [itex]\lambda[/itex]f

v

_{group}= d[itex]\omega[/itex]/dk

**3. The Attempt at a Solution**

dw/dk = [itex]\lambda[/itex]f - [itex]\lambda[/itex](d[itex]\lambda[/itex]f/d[itex]\lambda[/itex])

dw/dk = [itex]\lambda[/itex]f - [itex]\lambda[/itex]f

Not really sure where to go with this