-2cos2x-1=0
-2cosx-1=0 -2cosx=1 cosx=-1/2 x= ± arccos(-1/2)+2πn, n ∈ Z x= ± (π-arccos(1/2))+2πn, n ∈ Z x= ± (π - (π/3))+2πn, n ∈ Z [b]x= ± (2π/3)+2πn, n ∈ Z[/b]