bpt $\Leftrightarrow \frac{1}{2}.\frac{2.x!}{x!}-\frac{x!}{(x-2)!}\leq \frac{6}{x}.\frac{x!}{3!.(x-3)!}+10$ $\Leftrightarrow 1-\frac{(x-2)!.(x-1).x}{(x-2)!}\leq \frac{(x-3)!.(x-2).(x-1).x}{x.(x-3)!}+10$
$\Leftrightarrow 1-x.(x-1)\leq (x-2).(x-1)+10$
$\Leftrightarrow 1-x^{2}+x\leq x^{2}-3x+2+10$
$\Leftrightarrow 2x^{2}-4x+11\geq 0$ ( luôn đúng )
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