Then $du = \frac{1}{2} d\theta$, so $d\theta = 2du$.
When $\theta = 0$, $u = 0$.
When $\theta = \frac{3\pi}{2}$, $u = \frac{3\pi}{4}$.
Then the integral becomes
$\int_0^{\frac{3\pi}{4}} \sqrt{8} \cos^2 u \cdot 2 du = 2\sqrt{8} \int_0^{\frac{3\pi}{4}} \cos^2 u du$
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