已知sin(30°+α)=3/5,60°

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已知sin(30°+α)=3/5,60°
已知sin(30°+α)=3/5,60°<α<150°,求cosα的值

已知sin(30°+α)=3/5,60°
90

希望你能采纳我的意见,谢谢~sin(30°+α)=3/5,
60°<α<150°
所以30°+60°90°所以 a+30° 是第二象限角
cos(a+30°)<0
得 cos(30°+α)=-4/5

已知sin(30°+α)=3/5,60°<α<150°,
cos(30°+α)=-4/5,
cosa=cos[(30°+α)-30°]
=cos(30°+α)cos30°+sin(30°+α)sin30°
=-4/5*√3/2+3/5*1/2
=(3-4√3)/10