Let $f(x) = \cos \left( 2 \tan^{-1} \sin \left( \cot^{-1} \sqrt{\frac{1-x}{x}} \right) \right), 0 < x < 1$. Then:
\( (1-x)^{2} f'(x) + 2 \left( f(x) \right)^{2} = 0 \)
\( (1-x)^{2} f'(x) - 2 \left( f(x) \right)^{2} =0 \)
\( (1+x)^{2} f'(x) + 2 \left( f(x) \right)^{2} = 0 \)
\( (1+x)^{2} f'(x) - 2 \left( f(x) \right)^{2} = 0 \)