Question

cos 3x + cos x - cos 2x = 0

Answer 1

We have,

cos3x + cosx - cos2x = 0

Or 2cos(3x+x/2)cos(3x-x/2)-cos2x =0

Or 2cos2xcosx - cos2x =0

Or cos2x(2cosx-1) =0

So,

cos2x=0 or 2cosx-1=0

Or 2cosx = 1

Or cosx = 1/2

Or cosx = cos60° or cosπ/3

Or x = 60° or π/3