Ans : (D)
\(f(x)=\tan ^{-1}\left(\frac{1-2 \log x}{1+2 \log x}\right)+\tan ^{-1}\left(\frac{3+2 \log x}{1-6 \log x}\right)\)
let, \(2 \log x=\tan \theta\)
\(3=\tan \alpha\)
\(\therefore f(x)=\frac{\pi}{4}-\not+\alpha+\not\)
\(=\frac{\pi}{4}+\tan ^{-1}(3)=\) constant
\(\therefore \mathrm{f}^{\prime \prime}(\mathrm{x})=0\)