Answer
a.) $\displaystyle{V=2\pi\int_{0}^{\frac{\pi}{4}}\frac{\pi}{2}\tan x-x\tan x\ dx}\\ $
b.) $\displaystyle{V=2.25323}$
Work Step by Step
a.)
$\displaystyle{V=\int_{0}^{\frac{\pi}{4}}\left(2\pi\left(\frac{\pi}{2}-x\right)\right)\left(\tan x\right)\ dx}\\
\displaystyle{V=2\pi\int_{0}^{\frac{\pi}{4}}\frac{\pi}{2}\tan x-x\tan x\ dx}\\ $
b.)
$\displaystyle{V=2.25323}$