7. $\triangle A B C$ has an interior angle of $60^{\circ}, \sin 3 A+\sin 3 B+\sin 3 C=$
7. 0. $\sin 3 A+\sin 3 B+\sin 3 C=-4 \cos \frac{3 A}{2} \cos \frac{3 B}{2} \cos \frac{3 C}{2}=0$.

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7. 0. $\sin 3 A+\sin 3 B+\sin 3 C=-4 \cos \frac{3 A}{2} \cos \frac{3 B}{2} \cos \frac{3 C}{2}=0$.