Phương pháp giải

${\log _a}f\left( x \right) > {\log _a}b \Leftrightarrow \left\{ {\begin{array}{*{20}{l}}{f\left( x \right) > 0}\\{\left[ {\begin{array}{*{20}{l}}{\left\{ {\begin{array}{*{20}{l}}{a > 1}\\{f\left( x \right) > b}\end{array}} \right.}\\{\left\{ {\begin{array}{*{20}{l}}{0 < a < 1}\\{f\left( x \right) < b}\end{array}} \right.}\end{array}} \right.}\end{array}} \right..$