2cos^2a+sina=1 2(cosa)^2-1+sina=01-2(sina)^2+sina=02(sina)^2-sina-1=0sina=1(因为a在第三象限,所以舍）sina=-1/2cosa=-sqrt(3)/2sina/2=±sqrt[(1-cosa)/2]=±sqrt[(2+sqrt(3))/4]=±[(sqrt(2)/4+sqrt(6)/4]【sq...