Find the phase and group velocity of three plane waves...

frankR

Y_i(x,t) = A*Sin(k_ix - {&omega}_it), where i = 1, 2, 3



Phase velocity for two wave can be given by v = (&omega + [&omega]')/(k + k') and group velocity u = (&Omega - &omega')/(k - k') but what about three waves?

I'm not sure what to do for three waves. I've looked all over my text and all over the web and can't find anything, that's why I'm here, really stuck.

Edit: Forget what the code for greek characters is and can't find it anywhere on this site, sorry.