I have a question that I can't do...can anybody help me?

Investigate the convergence of the series (in the complex plane) for difference values of a and b:

Generalise your solution by determining what constraints are required on the values of a and b, so as to guarantee the convergence of the series?

$\displaystyle S(a,b) = \sum_{n=0}^{\infty}(a+bi)^n $

Thanks if you can help.