Thi HSG Toán 9!

T

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Áp dụng $3(A^2+B^2+C^2) \ge (A+B+C)^2$
$9(a^4+b^4+c^4) = 3 [3(a^4+b^4+c^4)] \ge 3(a^2+b^2+c^2)^2 = 3(a^2+b^2+c^2)(a^2+b^2+c^2) \ge (a+b+c)^2(a^2+b^2+c^2) = a^2+b^2+c^2 \ge ab +bc+ca$
 
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