$\lim_{x\rightarrow 0} (1+x^2)^{\cot^2 x}.$
Ta có $$(1+x^2)^{\cot^2 x} = e^{\cot^2 x \ln{(1+x^2)}}.$$
Hơn nữa,
$$\lim_{x\to0}\cot^2 x \ln{(1+x^2)}= \lim_{x\to0}\cos^2 x \frac{\ln{(1+x^2)}}{x^2}\frac{1}{\frac{\sin^2x}{x^2}}=1. $$
Suy ra $$\lim_{x\to0} (1+x^2)^{\cot^2 x} =e.$$