2: There is k in the denominator, which clearly points to one of the logarithmic expansions. Even the beginning of the indexing fits, the only problem is that there is "−2" instead of "−1" in the numerator and the power there does not fit. Fix it, 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.

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