8 Given $\triangle A B C$, $\tan ^{2} B=\tan A \tan C$, find the range of $B$.

In $\triangle A B C$, it is known that $\tan ^{2} B=\tan A \tan C$. Find the range of $B$.
$8 \tan B=-\tan (A+C)=\frac{\tan A+\tan C}{\tan A \tan C-1}=\frac{\tan A+\tan C}{\tan ^{2} B-1}$,
so
$$
\tan ^{3} B-\tan B=\tan A+\tan C .
$$

Notice that
$$
\tan A \tan C=\tan ^{2} B>0, A+C>0 .
$$

Therefore, $\tan B>0$. Removing the absolute value sign from the above equation, we get
$$
\tan ^{3} B-\tan B \geqslant 2 \tan B,
$$

which means
$$
\tan ^{3} B \geqslant 3 \tan B
$$

Thus, $\tan B \geqslant \sqrt{3}$, so $\frac{\pi}{3} \leqslant B<\frac{\pi}{2}$.