11. In $\triangle A B C$, the sides opposite to the internal angles $A, B, C$ are $a, b, c$ respectively. It is known that the area of $\triangle A B C$ is $\frac{a^{2}}{3 \sin A}$.
(1) Find $\sin B \sin C$;
(2) If $6 \cos B \cos C=1, a=3$, find the perimeter of $\triangle A B C$.