x+(1-x)=1,x∈(0,1)x(1-x)<=1/41/(x(1-x)>=4y=1/cos^2 θ + 1/(x(1-x)sin^2 θ)>= 1/cos^2 θ +4/sin^2 θ >= 5+2√4=9当x=1/2.tanθ=2成立