\(\frac{ab}{a+b}=\frac{bc}{b+c}=\frac{ac}{c+a}\Leftrightarrow\frac{a+b}{ab}=\frac{b+c}{bc}=\frac{c+a}{ac}\)
\(\Leftrightarrow\frac{1}{a}+\frac{1}{b}=\frac{1}{b}+\frac{1}{c}=\frac{1}{c}+\frac{1}{a}\)
\(\Leftrightarrow\hept{\begin{cases}\frac{1}{a}+\frac{1}{b}=\frac{1}{b}+\frac{1}{c}\\\frac{1}{b}+\frac{1}{c}=\frac{1}{c}+\frac{1}{a}\\\frac{1}{c}+\frac{1}{a}=\frac{1}{a}+\frac{1}{b}\end{cases}}\)
\(\Leftrightarrow\frac{1}{a}=\frac{1}{b}=\frac{1}{c}\Leftrightarrow a=b=c\)
Thay vào M được \(M=\frac{3a^2}{3a^2}=1\)
<br class="Apple-interchange-newline"><div id="inner-editor"></div>⇔1a +1b =1b +1c =1c +1a
⇔{
1a +1b =1b +1c |
1b +1c =1c +1a |
1c +1a =1a +1b |