证明：
1/sin^2 x+1/cos^2 x-1/tan^2 x
=(sin^2 x+cos^2 x)/sin^2 x+(sin^2 x+cos^2 x)/cos^2 x - 1/tan^2 x
=1+1/tan^2 x+1+tan^2 x-1/tan^2 x
=2+tan^2 x