Bài giải sai (từ dòng 2) vì $-1-1=-2$
$\lim\limits_{x\to 0}\dfrac{ \sqrt{1+x}-\sqrt{1-x} }{x}$
$=\lim\limits_{x\to 0}\dfrac{1+x-(1-x) }{x(\sqrt{1+x}+\sqrt{1-x}) }$
$=\lim\limits_{x\to 0}\dfrac{2}{\sqrt{1+x}+\sqrt{1-x}}$
$=\dfrac{2}{\sqrt1+\sqrt1}$
$=1$