Answer
$$\ln \left(1+\sqrt{2}\right)$$
Work Step by Step
\begin{align*}
\int_{0}^{\pi/4}\frac{dx}{\cos x}&=\int_{0}^{\pi/4}\sec xdx\\
&=\ln |\sec x+\tan x|\bigg|_{0}^{\pi/4}\\
&= \ln \left(1+\sqrt{2}\right)
\end{align*}
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