`x^2+2x+5 <= 4\sqrt{2x^2+4x+3}`
`<=>2x^2+4x+3+7 <= 8\sqrt{2x^2+4x+3}`
Đặt `\sqrt{2x^2+4x+3}=t` `(t > 0)`
Ptr có dạng: `t^2+7 <= 8t`
`<=>t^2-8t+7 <= 0`
`<=>1 <= t <= 7` (t/m)
`<=>1 <= \sqrt{2x^2+4x+3} <= 7`
`<=>1 <= 2x^2+4x+3 <= 49`
`<=>{(1 <= 2x^2+4x+3),(2x^2+4x+3 <= 49):}`
`<=>{(-2x^2-4x-2 <= 0 (LĐ)),(-1-2\sqrt{6} <= x <= -1+2\sqrt{6}):}`
`<=>-1-2\sqrt{6} <= x <= -1+2\sqrt{6}`