tìm các giới hạn sau:
a, \(\lim\limits_{x\rightarrow0}\frac{\sqrt{1+x^2}-1}{x}\)
b,\(\lim\limits_{x\rightarrow1}\frac{\sqrt[3]{x+7}-\sqrt{5-x^2}}{x-1}\)
c, \(\lim\limits_{x\rightarrow0}\frac{\sqrt[3]{1+x}-\sqrt[3]{1-x}}{x}\)
d, \(\lim\limits_{x\rightarrow2}\frac{\sqrt[3]{4x-2}}{x-2}\)