Question

已知sin（派+a）=2分1,计算cos（2派_a）

Answer 1

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.