Cho \(a > 0;b > 0\) 

B

- Hướng dẫn giải

Ta có: \({a^2} + {b^2} = 7{\rm{a}}b \Leftrightarrow {\left( {a + b} \right)^2} = 9ab\)

\(\Leftrightarrow \frac{{{{\left( {a + b} \right)}^2}}}{{{3^2}}} = ab \Leftrightarrow \log {\left( {\frac{{a + b}}{3}} \right)^2} = \log ab\)

\(2\log \frac{{a + b}}{3} = \log a + {\mathop{\rm logb}\nolimits}\)

\(\Leftrightarrow \log \frac{{a + b}}{3} = \frac{1}{2}\left( {\log a + \log b} \right)\)