ta có : x2(√9x4+7−3√8x3−1)=x2(√x4+7−2x+2x−3√8x3−1)=x2(9x4−4x2+7√9x4+7+8x3−8x3+14x2+2x3√8x3−1+3√(8x3−1)3)đến đây dễ rồi => $\mathop {\lim }\limits_{x \to +}x^2(\sqrt{9x^4+7}+\sqrt[3]{8x^3-1})=.........$
ta có : x2(√9x4+7−3√8x3−1)=x2(√x4+7−2x+2x−3√8x3−1)=x2(9x4−4x2+7√9x4+7+8x3−8x3+14x2+2x3√8x3−1+3√(8x3−1)3)đến đây dễ rồi => $\mathop {\lim }\limits_{x \to +\infty}x^2(\sqrt{9x^4+7}+\sqrt[3]{8x^3-1})=+\infty$