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a,b,c aparțin R*
a+b+c=0
[tex] \sqrt{ \frac{1}{ {a}^{2} } + \frac{1}{ {b}^{2} } + \frac{1}{ {c}^{2} } } = | \frac{1}{a} + \frac{1}{b} + \frac{1}{c} | [/tex]