Prove that \[ \sin \frac{\theta}{2} \sin \frac{7 \theta}{2}+\sin \frac{3 \theta}{2} \sin \frac{1...

Prove that
\[
\sin \frac{\theta}{2} \sin \frac{7 \theta}{2}+\sin \frac{3 \theta}{2} \sin \frac{11 \theta}{2}=\sin 2 \theta \sin 5 \theta
\]