\(\int\limits^{\frac{\pi}{4}}_0\frac{x}{\cos^2}dx=\int\limits^{\frac{\pi}{4}}_0x.d\left(\tan x\right)=x.\tan|^{\frac{\pi}{4}}_0-\int\limits^{\frac{\pi}{4}}_0\tan xdx=\frac{\pi}{4}+\ln\left(\cos x\right)|^{\frac{\pi}{4}}=\frac{\pi}{4}-\frac{1}{2}\ln2\)