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khonggilakhongthe hasn't added any friends yet.
28-01-2014 - 09:54
$\sum \left [ \left ( a+1 \right )^2+b^2+1 \right ]\geq \sum ( 2ab+2a+1 )$
===> $2S\leq \sum \frac{1}{ab+a+1}=\frac{1}{ab+a+1}+\frac{a}{ab+a+1}+\frac{ab}{ab+a+1}=1$ (vì $abc=1$)
$"="\Leftrightarrow a=b=c=1$