Question

已知tanθ=,θ∈(0,),求cos(-θ).

Answer 1

解：∵tanθ==,

且sin²θ+cos²θ=1,

θ∈(0,),sinθ＞0,cosθ＞0,

解得：sinθ=,cosθ=,

∴cos(-θ)=cos·cosθ+sin·sinθ

=.