[Toán 7]Khó

M

me0kh0ang2000

Theo BTĐ trong tam giác, ta có:

$a + b > c$ \Rightarrow $ ac + bc > c^2\ (1)$
$a + c > b$ \Rightarrow $ ab + bc > b^2\ (2)$
$b + c > a$ \Rightarrow $ ab + ac > a^2\ (3)$

Từ (1), (2), (3) \Rightarrow $(ac + bc + ab + bc + ab + ac) > ( a^2 + b^2 + c^2)$ \Rightarrow dpcm
 
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