sin(π+α)=-sinα=1/2.
∴ sinα=-1/2.
α=-30°.
或α=210°
cos(2π-α)=cosα.
cos(-30°)=cos30°=√3/2
∴cos(2π-α)=√3/2.
或cos(360°-210°)=cos210°
=cos(180°+30°)=-cos30°=-√3/2
∴cos(2π-α)=±√3/2.