come on.
If k=1, than the values are
π/6 + 2π/3 = 5π/6
-π/6 + 2π/3 = π/2
5π/6 + 2π/3 = 3π/2
-5π/6 + 2π/3 = -π/6
So with k=1 you get the values π/2, -π/6
The next values will be those + or - 2π/3, so check to see which ones lie in the desired interval.
What am I adding 2pi/3 to? Will i have 4 answers?