若cos(π+α)=－1/2,3π/2

若cos(π+α)=－1/2,3π/2
若cos(π+α)=－1/2,3π/2

若cos(π+α)=－1/2,3π/2
∵cos(π+α)=-cosα=-1/2 ∴coscos(π+α)=

cos (π+α)=-cosa=-1/2 cosa=1/2 sina=-√3/2 (3π/2<α<2π sina<0) sin(2π-α)=sin(-a)=-sina=√3/2