B2. Calculate $\sin (2 \alpha+\pi)$, if $\sin \alpha=\frac{5}{13}$ and $\alpha$ is an acute angle.
B2. Since $\alpha$ is an acute angle, $\cos \alpha=\sqrt{1-\sin ^{2} \alpha}=\frac{12}{13}$. Then we calculate $\sin (2 \alpha+\pi)=-\sin 2 \alpha=$ $-2 \sin \alpha \cos \alpha=-2 \cdot \frac{5}{13} \cdot \frac{12}{13}=-\frac{120}{169}$.

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