用a吧cosacosπ/6-sinasinπ/6-sina=3√3/5cosa*√3/2-sina*1/2-sina=3√3/5cosa*√3/2-sina*3/2=3√3/5两边除以√3cosa*1/2-sina*√3/2=3/5cosacosπ/3-sinasinπ/3=3/5所以cos(a+π/3)=3/5