\(\dfrac{1+cot^2x}{1-cot^2x}=\dfrac{1+\left(\dfrac{cosx}{sinx}\right)^2}{1-\left(\dfrac{cosx}{sinx}\right)^2}=\dfrac{sin^2x+cos^2x}{sin^2x}:\dfrac{sin^2x-cos^2x}{sin^2x}\)
\(=\dfrac{sin^2x+cos^2x}{sin^2x-cos^2x}=\dfrac{1}{-\left(cos^2x-sin^2x\right)}=\dfrac{-1}{cos2x}\)