A example in my textbook explain that $x^2-2x+2=0$ has the solutions $x = 1± i$. The distance between $x=0$ and $x = 1± i$ is $\sqrt{2}$ in the complex plane. So the radius of convergence of the Taylor series for $(x^2-2x+2)^{-1}$ is $\sqrt{2}$ around $x=0$. 

I looked for why this result is true, but I didn't find it. Is anyone could explain to me why this is true or just give me the theorem related?