bài khó quá

I

iceghost

Hình như $A=(2a+2b-c)^2+(2b+2c-a)^2+(2c+2a-b)^2$ thì phải
$=4a^2+4b^2+c^2+8ab-4ac-4bc +4b^2+4c^2+a^2 +8bc-4ab-4ca+4c^2+4a^2+b^2+8ac-4cb-4ab$ -_-
$=9a^2 + 9b^2 + 9c^2$
$=9(a^2+b^2+c^2)$
$=9m$
 
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