ta có : $x^2(\sqrt{9x^4+7}-\sqrt[3]{8x^3-1})=x^2(\sqrt{x^4+7}-2x+2x-\sqrt[3]{8x^3-1})=x^2(\frac{9x^4-4x^2+7}{\sqrt{9x^4+7}}+\frac{8x^3-8x^3+1}{4x^2+2x\sqrt[3]{8x^3-1}+\sqrt[3]{(8x^3-1)^3}})$đến đây dễ rồi
=> $\mathop {\lim }\limits_{x \to +\infty}x^2(\sqrt{9x^4+7}+\sqrt[3]{8x^3-1})=+\infty$