|
Từ $\frac{1}{a}+\frac{1}{b}+\frac{1}{c}=2 \Rightarrow (\frac{1}{a}+\frac{1}{b}+\frac{1}{c})^2=4$ $\Rightarrow \frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}+\frac{2}{ab}+\frac{2}{bc}+\frac{2}{ca}=4$ Vì $\frac{1}{a^2}+\frac{1}{b^2}+\frac{1}{c^2}=2 \Rightarrow \frac{2}{ab}+\frac{2}{bc}+\frac{2}{ca}=2$ $\Rightarrow \frac{1}{ab}+\frac{1}{bc}+\frac{1}{ca}=1 \Rightarrow \frac{a+b+c}{abc}=1 \Rightarrow a+b+c=abc$ (ĐPCM)
|