Taylor Series Radius
1. **Problem Statement:**
Find the Taylor series for the function $f$ centered at $6$ given that
$$f^{(n)}(6) = \frac{(-1)^n n!}{4^n (n+2)}$$
and determine the radius of convergence $R$ of this Taylor series.
2. **Taylor Series Formula:**
The Taylor series of $f$ centered at $a=6$ is
$$f(x) = \sum_{n=0}^\infty \frac{f^{(n)}(6)}{n!} (x-6)^n$$
3. **Substitute the given derivative formula:**
Substitute $f^{(n)}(6)$ into the series:
$$f(x) = \sum_{n=0}^\infty \frac{(-1)^n n!}{4^n (n+2)} \cdot \frac{(x-6)^n}{n!} = \sum_{n=0}^\infty \frac{(-1)^n}{4^n (n+2)} (x-6)^n$$
4. **Taylor series expression:**
$$\boxed{f(x) = \sum_{n=0}^\infty \frac{(-1)^n}{4^n (n+2)} (x-6)^n}$$
5. **Radius of Convergence $R$:**
Use the root test or ratio test. Consider the general term:
$$a_n = \frac{(-1)^n}{4^n (n+2)} (x-6)^n$$
Focus on the absolute value:
$$|a_n| = \frac{|x-6|^n}{4^n (n+2)}$$
Apply the root test:
$$\lim_{n \to \infty} \sqrt[n]{|a_n|} = \lim_{n \to \infty} \frac{|x-6|}{4} \sqrt[n]{\frac{1}{n+2}} = \frac{|x-6|}{4} \cdot 1 = \frac{|x-6|}{4}$$
For convergence, this limit must be less than 1:
$$\frac{|x-6|}{4} < 1 \implies |x-6| < 4$$
Thus, the radius of convergence is
$$\boxed{R = 4}$$