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Bài 1:
Ta có: $abc=a+b+c \Leftrightarrow \dfrac{1}{ab}+\dfrac{1}{ac}+\dfrac{1}{bc}=1$
Lại có:
$\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}=2$
$\Rightarrow \left(\dfrac{1}{a}+\dfrac{1}{b}+\dfrac{1}{c}\right)^2=4$
$\Leftrightarrow \dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}+2\left(\dfrac{1}{ab}+\dfrac{1}{ac}+\dfrac{1}{bc}\right)=4$
$\Rightarrow \dfrac{1}{a^2}+\dfrac{1}{b^2}+\dfrac{1}{c^2}=2$
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