f(θ)=[2sin^2(θ)+1]/sin2θ=[3sin^2(θ)+cos^2(θ）]/sin2θ= [3sin^2(θ)+cos^2(θ）]/（2sinθcosθ)=3cosθ/2sinθ+cosθ/2sinθ用均值不等式≥2√(3cosθ/2sinθ)(cosθ/2sinθ)=√3当且仅的当3cosθ/2sinθ=c...