設sinθ+cosθ=√2/3,π/2 sinθ^3是sinθ的3次

設sinθ+cosθ=√2/3,π/2 sinθ^3是sinθ的3次

由sinθ+cosθ=√2/3,平方後整理得sinθ*cosθ=-7/18,
(sinθ-cosθ)^2=(sinθ+cosθ)^2-4sinθ*cosθ=16/9,
π/2