1) Tìm giới hạn các hàm số sau:$a/ \mathop {\lim }\limits_{x \to 5}\frac{\sqrt{1+x}-\sqrt[3]{1+x}}{x}$
$b/ \mathop {\lim }\limits_{x \to 1}\frac{\sqrt[3]{x+7}-\sqrt{5-x^2}}{x-1}$
$c/ \mathop {\lim }\limits_{x \to 0}\frac{\sqrt{5-x}+\sqrt[3]{x^2+7}}{x^2-1}$