2cos²θ+(sinθ)^4-(cosθ)^4-1=2cos²θ-1+(sin²θ+cos²θ)(sin²θ-cos²θ)=cos2θ-(cos²θ-sin²θ)=cos2θ-cos2θ=0∴ 2cos²θ+(sinθ)^4=(cosθ)^4+1