Harmonic Progression (HP): Sequence whose reciprocals form AP. If 1/a, 1/b, 1/c, ... form AP, then a, b, c, ... form HP. General term: 1/hₙ = 1/h₁ + (n-1)d, so hₙ = 1/(1/h₁ + (n-1)d). Harmonic mean: For HP terms a, b, c with 2/b = 1/a + 1/c, then b is harmonic mean of a, c. Formula: H.M. = 2ac/(a+c). Example: HP 1/2, 1/5, 1/8, ... has d=1/10. H.M. of 2 and 5: 2(2)(5)/(2+5) = 20/7. Relationship: A.M. ≥ G.M. ≥ H.M. for positive numbers. Solving: Convert HP to AP, solve, convert back. Exam tip: Recognize HP from reciprocals forming AP. Practice: AM-GM-HM inequality problems.