tính 1.\mathop {\lim }\limits_{x \to 0}\frac{\sqrt[3]{1-3x}-\sqrt{1+2x}}{x}2.\mathop {\lim }\limits_{x \to 0}\frac{\sqrt{2x+1}-\sqrt[3]{x+1}}{\sqrt[3]{2x+1}-\sqrt{x+1}}3.\mathop {\lim }\limits_{x \to 0}\frac{\sqrt{x+1}-x^{2}-x-1}{\sqrt[3]{x+1}-2x-1}4.\mathop {\lim }\limits_{x \to 2}\frac{(\sqrt[3]{x+6}+1)(\sqrt{x+2}+1)-9}{x-2}5.\mathop {\lim }\limits_{x \to 2}\frac{\sqrt{x+2}+x-\frac{4}{\sqrt{x+2}}-2}{x^{3}-2x^{2}+x-2}