Tìm giới hạn$e)\mathop {\lim }\limits_{x \to 0}\frac{\cos^4 x -\sin^4 x -1}{\sqrt{x^2+1}-1 } $$f) \mathop {\lim }\limits_{x \to 0}\frac{\sin(\sin x)}{x} $$g) \mathop {\lim }\limits_{x \to 1}\frac{\sqrt{2x+1}-\sqrt[3]{x^2+1} }{\sin x} $$h)\mathop {\lim }\limits_{x \to 0}\frac{\sqrt{\cos x}-\sqrt[3]{\cos x} }{\sin^2 x} $