If a sequence is in AP, then the sequence obtained by taking the reciprocal of every term in the sequence forms an HP, that is if a,b,c,… form an AP, then 1a,1b,1c,… form an HP.
Let a,b,c form an HP. Then clearly, 1a,1b,1c form an AP.
b is called the Harmonic Mean.
Problems related to HP are generally solved by converting it into an AP.
Relationship between Arithmetic, Harmonic and Geometric Mean
AM∗HM=GM2
AM>GM>HM