sinθ+cosθ=根号2/3两边平方,得(sinθ+cosθ)²=2/9sin²θ+cos²θ+2sinθcosθ=2/9所以sin2θ=2sinθcosθ=2/9-1=-7/9则sin²2θ=49/81则cos²2θ=1-sin²2θ=32/81cos4θ =cos²2θ...