a/ \(\sqrt{a+b}=\sqrt{a+c}+\sqrt{b+c}\)
\(\Leftrightarrow a+b=a+c+b+c+2\sqrt{ab+ac+bc+c^2}\)
\(\Leftrightarrow-c=\sqrt{ab+ac+bc+c^2}\)
\(\Leftrightarrow c^2=ab+ac+bc+c^2\)
\(\Leftrightarrow ab+ac+bc=0\)
\(\Leftrightarrow ab=-c\left(a+b\right)\)
\(\Leftrightarrow\frac{ab}{a+b}=-c\)
\(\Leftrightarrow\frac{a+b}{ab}=-\frac{1}{c}\)
\(\Leftrightarrow\frac{1}{a}+\frac{1}{b}=-\frac{1}{c}\)
\(\Leftrightarrow\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=0\)(đúng)