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
∴ cos(α-π/4)
=cosα*cos(π/4)+sinαsin(π/4)
=(-3/5)*(√2/2)+(4/5)*(√2/2)
=√2/10cos(α-400°)cos(20°+α)-sin(α-40°)sin(200°+α)=新问题请采纳后重新求助。