Step 1:
First, we rewrite the integrand using the identity $\cos^2 \theta = \frac{1}{2}(\cos 2\theta + 1)$:
\[\int_{0}^{1} \mathrm{dr} \int_{0}^{\pi / 4} r \cos ^{2} \theta \mathrm{d} \theta = \frac{1}{2} \int_{0}^{1} \mathrm{dr} \int_{0}^{\pi / 4} r (\cos 2\theta
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