Ta có : \(a-\sqrt{ab}-6b=0\)
\(\Leftrightarrow a+2\sqrt{ab}-3\sqrt{ab}-6b=0\)
\(\Leftrightarrow\sqrt{a}\left(\sqrt{a}+2\sqrt{b}\right)-3\sqrt{b}\left(\sqrt{a}+2\sqrt{b}\right)=0\)
\(\Leftrightarrow\left(\sqrt{a}+2\sqrt{b}\right)\left(\sqrt{a}-3\sqrt{b}\right)=0\)
\(\Leftrightarrow\sqrt{a}-3\sqrt{b}=0\) ( Vì \(a,b>0\) )
\(\Leftrightarrow a=9b\)
\(P=\dfrac{a+b}{a+\sqrt{ab}+b}=\dfrac{9b+b}{9b+\sqrt{9b^2}+b}=\dfrac{10b}{13b}=\dfrac{10}{13}\)