cm bdt

E

eye_smile

$A={a^2}+{b^2}+{c^2}+{d^2}+ab+cd+bc+ad+ac+ac+bd+bd$ \geq $2\sqrt{{a^2}{b^2}{c^2}{d^2}}+2\sqrt{abcd}+2\sqrt{abcd}+2\sqrt{2ac.2bd}=12$
Dấu "=" xảy ra \Leftrightarrow a=b=c=d=1

 
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