Convergance series

[chengeto]

Guys l don't know where to start on this question. Can you please give me pointers as to which path to follow /hints.

 

[galactus]

Here's a hint:

Suppose that \(\displaystyle \sum{|C_{k}|}\) converges, then \(\displaystyle \lim_{k\to {+\infty}}|C_{k}|=0\)