I'm given the problem cos4x - cos2x = 0. It looks simple, but I I don't know what to do with 4x!
You can transform it by using the duplication formula : cos(4x) = cos(2*2x)
Then, you'll explain cos(4x) in terms of cos(2x) and you'll just have to solve an equality with cos(2x) as the variable.
Remember also that if cos(x)=cos(a), x=a + 2pi or x= -a + 2pi