已知sinθ=-4/5,tanθ>0,求cos(θ+π/4)的值
问题描述：
已知sinθ=-4/5,tanθ>0,求cos(θ+π/4)的值
答
sinθ=-4/5,tanθ>0
cosθ=-3/5
cos(θ+π/4)
=cosθ.cos(π/4)-sinθ.sin(π/4)
=-(3/5)(√2/2) +(4/5)(√2/2)
=√2/10