分类：教育/科学-学习帮助
问题：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
回答：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