sina/cosa=1/2 ,2sina=cosa ,4sin²a=cos²a①
sin²a+cos²a=1②
联立①②,可得sina=±根号5/5 ,cosa=±2根号5/5
∵cos(a+π/3)为第4象限角 ∴a为第三象限角
∴sina=-根号5/5 ,cosa=-2根号5/5
cos(a+π/3)=cosacosπ/3-sinasinπ/3
代入可得cos(a+π/3)=根号15/10-2根号5/10
这题第四象限角指代不明,我是把它当成cos(a+π/3)的条件来算