Bài này chắc khỏi đạo hàm:
\(y=\dfrac{3\sqrt{2}.2.\dfrac{\sqrt{2}}{12}x+\dfrac{\sqrt{2}}{3}.2.\dfrac{3\sqrt{2}}{4}\sqrt{1+9x^2}}{8x^2+1}\le\dfrac{3\sqrt{2}\left(\dfrac{1}{72}+x^2\right)+\dfrac{\sqrt{2}}{3}\left(\dfrac{9}{8}+1+9x^2\right)}{8x^2+1}\)
\(y\le\dfrac{\sqrt{2}\left(6x^2+\dfrac{3}{4}\right)}{8x^2+1}=\dfrac{3\sqrt{2}}{4}.\dfrac{8x^2+1}{8x^2+1}=\dfrac{3\sqrt{2}}{4}\)
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