Solve \int_{0}^{x / 4} \tan ^{2} x d x

\tan ^{2} \mathrm{x}=\sec ^{2} \mathrm{x}-1
\Rightarrow \int_{0}^{\pi / 4}-1 \mathrm{dx}+\int_{0}^{\pi / 4}\left(\sec ^{2} \mathrm{x}\right) \mathrm{dx}
=-\pi / 4+[\tan \mathrm{x}\}_{0}^{\pi / 4}
=1-\pi / 4