cos(π/6-θ)=a
cos(5π/6+θ)
=-cos(π-5π/6-θ)
=-cos(π/6-θ)
=-a
sin(2π/3-θ)
=sin(π-2π/3+θ)
=sin(π/3+θ)
=cos(π/2-π/3-θ)
=cos(π/6-θ)
=a