Chứng minh

E

eye_smile

Ta có: $A=2a^2.b^2+2b^2.c^2+2c^2.a^2-a^4-b^4-c^4$
$=4b^2.c^2-(a^2-b^2-c^2)^2$
$=(2bc-a^2+b^2+c^2).(2bc+a^2-b^2-c^2)$
$=[(b+c)^2-a^2].[a^2-(b-c)^2]$
$=(b+c-a).(b+c+a).(a-b+c).(a+b-c)>0$
 
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