Giải thích các bước giải:
Bài 5:
$\dfrac{1}{c}=\dfrac{1}{2}(\dfrac{1}{a}+\dfrac{1}{b})$
$\dfrac{2}{c}=\dfrac{a+b}{ab}$
$\rightarrow 2ab=ca+cb$
$\rightarrow ab-ca=cb-ab$
$\rightarrow a(b-c)=b(c-a)$
$\rightarrow\dfrac{a}{b}=\dfrac{c-a}{b-c}$
$\rightarrow\dfrac{a}{b}=\dfrac{a-c}{c-b}$