9.1.2. Find the minimum value of the function $f(x)=\operatorname{tg}^{2} x-4 \operatorname{tg} x-12 \operatorname{ctg} x+9 \operatorname{ctg}^{2} x-3$ on the interval $\left(-\frac{\pi}{2} ; 0\right)$.
Answer: $3+8 \sqrt{3} \approx 16.86$.