[SOLVED] Superposition of two waves

I'm strugggling with what should be an easy question;

I have a 2D free-surface water wave with surface displacement as follows,

$\displaystyle \eta = Acos(Kx - \omega t)$

Now what happens when you superimpose another wave of the same amplitude, but with a frequency of $\displaystyle \omega + \Delta\omega$ and wavenumber $\displaystyle K + \Delta K$?

I thought that you should get an amplitude-modulated wave, but I'm not sure how to show this...

Help anyone? Thanks in advance!