(1+cot^2x)/(1-cot^2x)……分子分母同乘以sin^2x
=( sin^2x +cos^2x)/( sin^2x - cos^2x)
=1/( sin^2x - cos^2x)
=1/[ sin^2x –(1- sin^2x)]
=1/(2sin^2x-1)
∴原式成立.