a∈(3π /2 , 2π)a在第四象限∴sina<0sina=-√(1-cos^2a)=-4/5cos(a+π/3)=cosacosπ/3-sinasinπ/3=3/5*1/2-(-4/5)*√3/2=3/10+4√3/10=(3+4√3)/10
(3-根号30）/10
