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 "+1" in the exponent, then look up the appropriate expansion.

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 f (x) and then manipulate simultaneously both sides of this equality.

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