<s>[INST] Is there any way to evaluate this limit without applying de l'Hôpital rule nor series expansion? Is there a way to evaluate this limit:
$$\lim_{x \to 0}  \frac{\sin(e^{\tan^2 x} - 1)}{\cos^{\frac35}(x) - \cos(x)}$$
without using de l'Hôpital rule and series expansion?  
Thank you,
 [/INST] You should know the following limits:
$$\begin{split}\lim_{y\to 0} \frac{\sin y}{y} &= 1\\ \lim_{y\to 0} \frac{e^y-1}{y} &= 1\\ \lim_{y\to 0} \frac{\tan y}{ y} &= 1\\ \lim_{y\to 0} \frac{(1+y)^\theta -1}{y} &= \theta \qquad \text{(}\forall \theta \in \mathbb{R}\text{)}\\
\lim_{y\to 0} \frac{1-\cos y}{y^2} &= \frac{1}{2}\end{split}$$
which can be proved using only elementary Calculus tools (i.e. without any Differential Calculus technique).
These five limits are usually written as asymptotic relations in the following manner:
$$\tag{1} \sin y \approx y$$
$$\tag{2} e^y-1 \approx y$$
$$\tag{3} \tan y \approx y$$
$$\tag{4} (1+y)^\theta -1 \approx \theta\ y$$
$$\tag{5} 1-\cos y \approx \frac{1}{2}\ y^2$$
as $y\to 0$. Using asymptotics (1) - (5) you find:
$$\begin{split} \sin(e^{\tan^ 2 x} - 1) &\approx e^{\tan^2 x}-1 &\quad \text{by (1)}\\
&\approx \tan^2 x &\quad \text{by (2)}\\
&\approx x^2  &\quad \text{by (3)}\end{split}$$
$$\begin{split}\cos^{3/5}(x) - \cos(x) &= \Big(\big(1+(\cos x-1)\big)^{3/5} -1\Big) + \Big(1-\cos x\Big)\\ &\approx \frac{3}{5}\ (\cos x-1) + (1-\cos x) &\text{by (4) with } \theta =3/5\\
&= \frac{2}{5}\ (1-\cos x)\\
&\approx \frac{1}{5}\ x^2 &\text{by (5)}\end{split}$$
hence:
$$\lim_{x \to 0} \frac{\sin(e^{\tan ^ 2 {x}} - 1)}{\cos^{\frac{3}{5}}(x) - \cos(x)} = \lim_{x\to 0} \frac{x^2}{\frac{1}{5}\ x^2}=5\; .$$
</s>