已知Sinα=15/17,α∈(π/2,π),求cos(Π/3+α),cos(π/3－α)
问题描述：
已知Sinα=15/17,α∈(π/2,π),求cos(Π/3+α),cos(π/3－α)
答
郭敦顒回答：
∵Sinα=15/17,α∈(π/2,π),
∴α=118.072°,
又Π/3=60°,
∴cos(Π/3+α)= cos（60°+118.072°）= cos178.072°=－0.9994.
cos(π/3－α)= cos（118.072°－60°）= cos58.072°=0.5288.