Harmonic mean for a non-empty collection of numbers "a", "a", …, "a", all different from 0, is defined as the reciprocal of the arithmetic mean of the reciprocals of the "a"s:

One example where it is useful is calculating the average speed for a number of fixed-distance trips. For example, if the speed for going from point "A" to "B" was 60 km/h, and the speed for returning from "B" to "A" was 40 km/h, then the average speed is given by

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