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E

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BĐT \Leftrightarrow $a^2+b^2+ab-2013(a+b) \ge 0$

\Leftrightarrow $a(a-1342)+b(b-1342)+ab-671(a+b) \ge 0$

Lại có: $(a-1342)(b-1342) \ge 0$

\Leftrightarrow $ab \ge 1342(a+b)-1342^2$

\Leftrightarrow $ab-671(a+b) \ge 671(a+b)-1342^2 \ge 671.2.1342-1342^2=0$

\Rightarrow đpcm
 
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