\( a=\displaystyle\lim_{n\to\infty}\sum_{k=1}^{n}\dfrac{2n}{n^{2}+k^{2}} \) and \( f(x)=\sqrt{\dfrac{1-\cos x}{1+\cos x}},\, x\in(0,1), \) then:
\( 2\sqrt{2}\,f\!\left(\dfrac{a}{2}\right)=f'\!\left(\dfrac{a}{2}\right) \)
\( f\!\left(\dfrac{a}{2}\right)f'\!\left(\dfrac{a}{2}\right)=\sqrt{2} \)
\( \sqrt{2}\,f\!\left(\dfrac{a}{2}\right)=f'\!\left(\dfrac{a}{2}\right) \)
\( f\!\left(\dfrac{a}{2}\right)=\sqrt{2}\,f'\!\left(\dfrac{a}{2}\right) \)