Ta có:
$\dfrac{1}{a} +\dfrac{1}{b}+\dfrac{1}{c}= 2\\
<=> (\dfrac{1}{a} +\dfrac {1}{b}+\dfrac{1}{c})^2= 4\\
<=> \dfrac{1}{a^2} + \dfrac{1}{b^2} +\dfrac{1}{c^2}+2\dfrac{1}{ac}+2\dfrac{1}{bc}+2\dfrac{1}{ab}=4\\
<=> \dfrac{1}{a^2} + \dfrac{1}{b^2} +\dfrac{1}{c^2} +2(\dfrac{1}{ac}+\dfrac{1}{bc}+\dfrac{1}{ab})= 4\\
<=> \dfrac{1}{a^2} + \dfrac{1}{b^2} +\dfrac{1}{c^2}+ 2(\dfrac{b+a+c}{abc})=4\\
<=> \dfrac{1}{a^2} + \dfrac{1}{b^2} +\dfrac{1}{c^2}+ 2(\dfrac{abc}{abc})=4\\
<=> \dfrac{1}{a^2} + \dfrac{1}{b^2} +\dfrac{1}{c^2} +2=4\\
<=> \dfrac{1}{a^2} + \dfrac{1}{b^2} +\dfrac{1}{c^2} =2 (dpcm) $