## Calculus 8th Edition

$R=\frac{1}{c}$
$\lim\limits_{n \to \infty} \sqrt[n] {|c_{n}x^{n}|}=|x|\lim\limits_{n \to \infty} \sqrt[n] {|c_{n}|}$ $=c|x|\lt 1$ for convergence, or $|x|\lt 1/c$ so we will get $R=\frac{1}{c}$