(1) sin(a/2)/1=(1/4)/cos(a/2)sina=1/2a=30°（2）cos(a-y)=cos(30°-y)=-1/7cos30°cosy+sin30°siny=-1/7siny=-√3cosy-2/7两边平方 1-cos^2y=3cos^2y+(4√3/7)cosy+4/494cos^2y+(4√3/7)cosy-45/49(2cosy+5√3...