Harmonic progression and HP nth term (Reciprocals form an AP; nth term is 1 over (AP nth term in reciprocals))

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Reciprocals form an AP; nth term is 1 over (AP nth term in reciprocals)

What each symbol means

SymbolWhat it stands for
AFirst term of reciprocal AP
DCommon difference of reciprocal AP

When to use this

All terms non-zero; denominators must not vanish.