数学题老师谢谢了1,cos(α-400°)cos(20°+α)-sin(α-40°)sin(200°+α)=?2,已知c

/>（1）那个问题我没有做错啊.是你的题目的选项设置有误
1,cos(α-400°)cos(20°+α)-sin(α-40°)sin(200°+α)
=cos(α-40°-360°)cos(20°+α)-sin(α-40°)sin(180°+20°+α)
=cos(α-40°)cos(20°+α)+sin(α-40°)sin(20°+α)
=cos(α-40°-20°-α)
=cos(-60°)
=1/2
2. cosα=-3/5,α属于(π/2,2π)?应该是α属于(π/2,π)吧,否则范围就没有意义了sinα>0又sin²α=1-cos²α=1-(-3/5)²=16/25∴ sinα=4/5
sinB=-12/13, B是第三象限角,则cosB=-5/13∴ cos(B-α) =cosBcosα+sinB*sinα =(-5/13)*(-3/5)+(-12/13)*(4/5) =-63/65