cosacosπ/6+sinasinπ/6+sina=4/5√3√3/2cosa+1/2sina+sina=4/5√3√3/2cosa+3/2sina=4/5√31/2cosa+√3/2sina=4/5∴sin(π/6+a)=4/5∴cos(a+2π/3)=cos[(a+π/6)+π/2]=-sin(a+π/6)=-4/5