cos(π+a)=-1/2-cosa=-1/2cosa=1/23π/2＜a＜2πa=5π/3sin(2π+a)=sina=sin5π/3=-√3/2
cos(π+a)=-cosa =-1/2 cosa =1/2 3π/2＜a＜2π sina>0sin(2π+a)=sina=√3/2
