how to calculate $E(1/n_h) in poststratification [on hold]

I don’t know how to apply Taylor expansion to approximate E(1/nh)E(1/n_h)

nhn_h : number of units selected in the sample from hth stratum.

Wh=Nh/NW_h = N_h/N : stratum weight

I know the approximation is 1/(nWh)+(1−Wh)/(n2W2h)1/(nW_h)+(1-W_h)/(n^2W_h^2)

help me

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