Limit of the form $0/0$ How do I find $$\lim_{x \to 0}\frac{\cos(\sin x)-\cos x}{x^4}$$ I tried rewriting $\cos$ in the form of $\sin$ and then applying $\sin x/x$ form, but it doesn't work out. Does it?

First use Taylor series $$\sin x=x-\frac{x^3}{6}+o(x^3)$$ to get $$\lim_{x\to 0}\frac{x-\sin x}{x^3}=\frac{1}{6}$$ Next we can proceed as follows
\begin{align}
L&=\lim_{x\to 0}\frac{\cos \sin x - \cos x}{x^4}\notag\\
&=\lim_{x\to 0}\dfrac{2\sin\left(\dfrac{x+\sin x}{2}\right)\sin\left(\dfrac{x-\sin x}{2}\right)}{x^4}\notag\\
&=\lim_{x\to 0}2\cdot\dfrac{\sin\left(\dfrac{x+\sin x}{2}\right)}{\dfrac{x+\sin x}{2}}\cdot\dfrac{x+\sin x}{2x}\notag\\
&\,\,\,\,\dfrac{\sin\left(\dfrac{x-\sin x}{2}\right)}{\dfrac{x-\sin x}{2}}\cdot\dfrac{x-\sin x}{2x^3}\notag\\
&=2\cdot 1\left(\frac{1}{2}+\frac{1}{2}\right)1\cdot\frac{1}{12}=\frac{1}{6}\notag
\end{align}
