Question: How i can find thus result by applying the long wave limit?

Before at most a year i asked about this applying long wave limit which is coming from some kind of series which i am not sure to how get this result from the series but i provide just the two function of series for appyting the long wave limit and then we can generalized to N value of them, i asked to Ai too he give me a good explanation and somehow he did programing but  i am not cool with programing of Ai,  i  am looking for result eq14 and eq 20 , i have a series for eq(12) N=2 and for other N too but by applying long wave limit it is become to eq(14) and then F[4] becone to eq(20) i want that result by don't have any idea how reach it  

Long-wave-limit-testing.mw

Ai-result.mw

update F4

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