<s>[INST] $f(x)=\frac{\sin x}{x}$, prove that $|f^{(n)}(x)|\le \frac{1}{n+1}$ This is a homework question.

Let $f: (0, \infty) \to \mathbb{R}$ with $f(x)=\dfrac{\sin x}{x}$. I
  have to prove that 
$$|f^{(n)}(x)|\le \frac{1}{n+1}$$ 
where $f^{(n)}$ is the nth derivative of $f$.

I started to do a few derivatives:
$f^{(1)}(x)=\dfrac{1}{x^2}(x \cos x-\sin x)$
$f^{(2)}(x)=\dfrac{1}{x^3}((2-x^2) \sin x-2x\cos x)$
$f^{(3)}(x)=\dfrac{1}{x^4}(3(x^2-2) \sin x-x(x^2-6)\cos x)$
$f^{(4)}(x)=\dfrac{1}{x^5}(4x(x^2-6)\cos x + (x^4-12x^2+24)\cos x)$
I noticed that in denominator, there is always $x^{n+1}$ (because there is $x$ in denominator of $f$), but I couldn't spot a pattern for the numerator that would help me prove the inequality. Can I get a hint or a clue, please?
 [/INST] I will assume (since it's not specified) that $n$ is a non-negative integer.
Claim: For $f:(0,\infty)\to \mathbb{R}$, defined by $f(x)=\dfrac{\sin x}{x}$ and any non-negative integer $n$, we have:
$$f^{(n)}(x)=\frac{1}{x^{n+1}}\int_0^xu^n\cos\left(u+\frac{n\pi}{2}\right)\,du$$
Proof: I will prove this by induction. The case $n=0$ is obvious:
$$f^{(0)}(dx)=f(x)=\frac{\sin x}{x}=\frac{1}{x}\int_0^x\cos u\,du$$
Now assume it is true for some positive integer $n \geq 1$. Then, integrating by parts:
$$
\begin{aligned}
f^{(n+1)}(x)&=(f^{(n)})'(x)\\
&=\frac{1}{x^{n+2}}\left(-(n+1)\int_0^xu^n\cos\left(u+\frac{n\pi}{2}\right)+x\cdot x^n\cos\left(x+\frac{n\pi}{2}\right)\right)\\
&=\frac{1}{x^{n+2}}\left[-u^{n+1}\cos(u+\frac{n\pi}{2})\bigg|_0^x+\int_0^xu^{n+1}(\cos\left(u+\frac{n\pi}{2}\right))'\,du+x^{n+1}\cos(x+\frac{n\pi}{2})\right] \\
&=\frac{1}{x^{n+2}}\int_0^xu^{n+1}\cos\left(u+\frac{(n+1)\pi}{2}\right)\,du
\end{aligned}
$$
Claim proved. Now, the inequality follows immediately:
$$|f^{(n)}(x)|=\left|\frac{1}{x^{n+1}}\int_0^xu^n\cos\left(u+\frac{n\pi}{2}\right)\,du\right|\leq \frac{1}{x^{n+1}}\int_0^xu^n\,du=\frac{1}{n+1}$$
</s>