[QUOTE=princeps;708543]$\displaystyle \log_{b}\sqrt {\frac{4x^9}{z^8}}=\frac{1}{2} \cdot \log_{b} \frac{4x^9}{z^8}=\frac{1}{2} \cdot(\log_{b}4x^9-\log_{b}z^8)=$
$\displaystyle =\frac{1}{2} \cdot (\log_{b} 4+9\log_{b} x-8 \log_{b}z)$[/QUOTE
Thanks for correcting my mistake