sin2θa+cosθ+sin2θa−cosθ=asin2θ−sin2θcosθ+asin2θ +cosθsin2θ(a+cosθ)(a−cosθ)=2asin2θa2−cos2θ因为tanθ=1−aa，所以2asin2θa2−cos2θ=2atan2θa2tan2θ+a2−1=2a×1−aaa2×1−aa+a2−1...