Do a,b,c không thể = 0 nên ta có$:\frac{a+2b}{ab}=\frac{5}{2}\Leftrightarrow \frac{1}{b}+\frac{2}{a}=\frac{5}{2}$
tương tự2 cái kia$:\Rightarrow3(\frac{1}{b}+\frac{1}{a}+\frac{1}{c})=\frac{11}{2}\Rightarrow 3(\frac{ab+bc+ca}{abc})=\frac{11}{2}$
$\Rightarrow \Rightarrow \frac{abc}{ab+bc+ca}=\frac{6}{11}$