$\displaystyle f(x)=\frac{1}{4+x^2}$

a)write the first four terms and general term for the power series expansion of f about x=0

b)find the radius of convergence of the series found in part a)

Sorry but i don't really know how to do power series problems... I know

$\displaystyle \sum_{0}^{\infty}C_{n}X^n=C_{0}+C_{1}X+C_{2}X^2...$ but i don't really know what this means. Any Help?