The harmonic mean of the positive real numbers a1,...,an is defined to be
In certain situations, the harmonic mean provides the correct notion of "average". For instance, if for half the distance of a trip you travel at 40 miles per hour and for the other half of the distance you travel at 60 miles per hour (i.e. less time), then your average speed for the trip is given by the harmonic mean of 40 and 60, which is 48. Similarly, if in an electrical circuit you have two resistors connected in parallel, one with 40 ohms and the other with 60 ohms, then the average resistance is 48 ohms (i.e. if you replace each resistor by a 48 ohm resistor, the total resistance will stay the same). Typically the harmonic mean is appropriate for situations when the average of a rate is desired.
A "simple" formula for the harmonic mean of two numbers is to multiply the two numbers, and divide that quantity by the arithmetic mean of the two numbers. In mathematical terms:
This is essentially the same formula as above, only it is simpler for those people who, for example, want to write it as a calculator program.