$\int_{0}^{\frac{\pi}{4}} \frac{\cos \theta}{\cos ^{2} \theta} d \theta=?$

$\begin{array}{l} \mathrm{I}=\int_{0}^{\frac{\pi}{4}} \frac{\cos \theta}{\cos ^{2} \theta} \mathrm{d} \theta=\int_{0}^{\frac{\pi}{4}} \frac{1}{\cos \theta} \mathrm{d} \theta=\int_{0}^{\frac{\pi}{4}} \sec \theta \mathrm{d} \theta=[\ln |\tan \theta+\sec \theta|]_{0}^{\frac{\pi}{4}} \\ =\ln \left|\tan \frac{\pi}{4}+\sec \frac{\pi}{4}\right|-\ell \mathrm{n}|\tan 0+\sec 0|=\ell \mathrm{n}|1+\sqrt{2}|-\ell \mathrm{n} 1=\ell \mathrm{n}|1+\sqrt{2}|-0\end{array}$