Cosxcos3x-cos(x+3x)=0
cosxcos3x-cosxcos3x+sinxsin3x=0
sinxsin3x=0
sinx=0⇒x=πn,n∈z
0≤πn≤2π
0≤n≤2
n=0⇒x=0
n=1⇒x=π
n=2⇒x=2π
sin3x=0⇒3x=πk,k∈z⇒x=πk/3,k∈z
0≤πk/3≤2π
0≤k≤6
k=0⇒x=0
k=1⇒x=π/3
k=2⇒x=2π/3
k=3⇒x=π
k=4⇒x=4π/3
k=5⇒x=5π/3
k=6⇒x=2π
x={π/3;2π/3;π;4π/3;5π/3;2π}