cos3θ-cosθ=0 θ∈[0,π]cos3θ-cosθ=cos(2θ+θ)-cosθ=cos2θcosθ-sin2θsinθ-cosθ=(cos2θ-1)cosθ-sin2θsinθ=(1-2sin^2θ-1)cosθ-2sinθcosθsinθ=-2sin^2θcosθ-2sin^2θcosθ=-4sin^2θcosθ=0所以...