9: There is k in the denominator, which clearly
points to one of the logarithmic expansions. However, some adjustments need
to be done first. Get rid of the extra 2 in the denominator and of the
Some people are not exactly familiar with logarithmic expansions. Those
might prefer another approach. The k in the denominator disappears
when the series gets differentiated, then it becomes a series that should be
approachable using the geometric expansion. However, for the differentiation
to work, one has to first fix the exponent to read k.
All this is
usually handled more easily if you first denote the sum of the given series as