Finding group velocity and Phase velocity

[vivekrai]

Homework Statement

A wave packet in a dispersive medium is given as :
y(t) =cos(x-5t)cos(.2x-.4t)cos(.1x-.2t)
Find group velocity and phase velocity for the wave packet. Hence plot w-k variation for the calculated values.

2. The attempt at a solution

We know that for the wave, Vg = dw/dk and Vp=w/k.

I am clueless on how to proceed from here. Does the cosine term with lowest 'w' acts as the the modulating part? Please enlighten me with the correct information. It's not homework question, I'm trying this on myself so I may miss some critical information on how to approach it. Thanks! :)

 

[vivekrai]

Bump Please!