Khi đó l o g a b + 1 4 l o g b a bằng
Giải thích
Ta có \(\frac{1}{{{\rm{lo}}{{\rm{g}}_{ab}}a}} + \frac{1}{{{\rm{lo}}{{\rm{g}}_{\sqrt[4]{{ab}}}}b}} = \frac{9}{4} \Leftrightarrow {\rm{lo}}{{\rm{g}}_a}\left( {ab} \right) + \frac{1}{4}{\rm{lo}}{{\rm{g}}_b}\left( {ab} \right) = \frac{9}{4}\)
\( \Leftrightarrow 1 + {\rm{lo}}{{\rm{g}}_a}b + \frac{1}{4}\left( {1 + {\rm{lo}}{{\rm{g}}_b}a} \right) = \frac{9}{4}\)\( \Leftrightarrow {\rm{lo}}{{\rm{g}}_a}b + \frac{1}{4}{\rm{lo}}{{\rm{g}}_b}a = 1\). Chọn A.