Suppose you want to find the radius of convergence of the power series $ \sum\limits_{n = 0}^\infty {( - 1)^n \frac{{2^n }}{{n!}}x^n } . $ Let $A_n = ( - 1)^n \frac{{2^n }}{{n!}}x^n$, and use the ratio test: $ \mathop {\lim }\limits_{n \to \infty } \bigg|\frac{{A_{n + 1} }}{{A_n }}\bigg| = \mathop {\lim }\limits_{n \to \infty } \Bigg|\frac{{\frac{{( - 1)^{n + 1} 2^{n + 1} x^{n + 1} }}{{(n + 1)!}}}}{{\frac{{( - 1)^n 2^n x^n }}{{n!}}}}\Bigg| = \mathop {\lim }\limits_{n \to \infty } \bigg|\frac{{ - 2x}}{{n + 1}}\bigg| = 0. $ From this conclude that the series converges for all $x \in \mathbb{R}$, and hence the radius of convergence is $\infty$.
EDIT: Note that $ \sum\limits_{n = 0}^\infty {( - 1)^n \frac{{2^n }}{{n!}}x^n } = \sum\limits_{n = 0}^\infty {\frac{{( - 2x)^n }}{{n!}}} = e^{ - 2x} . $