高一正弦和角公示试题已知sin(30°+α）＝３／５,60°&l
已知sin(30°+α）＝３／５,60°<α<150°，求cα的值
已知sin(30°+α）＝3/5, 60°<α<150°，求cosα的值 60°<α<150°--->90°<30°+α<180° sin(30°+α）＝3/5---->cos(30°+α)=-4/5 cosα=cos[(30°+α)-30°] =cos(30°+α)cos30°+sin(30°+α)sin30° =(-4/5)(√3/2)+(3/5)(1/2) =(3-4√3)/10