9.1.4. Find the minimum value of the function $f(x)=\operatorname{tg}^{2} x+2 \operatorname{tg} x+6 \operatorname{ctg} x+9 \operatorname{ctg}^{2} x+4$ on the interval $\left(0 ; \frac{\pi}{2}\right)$.
Answer: $10+4 \sqrt{3} \approx 16.93$