Harmonic mean
The harmonic mean (frequently abbreviated HM) is a special kind of mean (like arithmetic mean and geometric mean). The harmonic mean of a set of 
positive real numbers 
is defined to be: 
.
The restriction to positive numbers is necessary to avoid division by zero. For instance, if we tried to take the harmonic mean of the set 
we would be trying to calculate 
, which is obviously problematic.
The harmonic mean is a part of a frequently used inequality, the Arithmetic mean-Geometric mean-Harmonic mean inequality. The Inequality states that for a set of positive numbers 
: ![$\frac{x_1+x_2+\ldots+x_n}{n}\ge \sqrt[n]{x_1\cdot x_2 \cdots x_n}\ge \frac{n} {\frac{1}{x_1}+\frac{1}{x_2}+...+\frac{1}{x_n}}$](http://latex.artofproblemsolving.com/3/4/8/3480680951e4be70f6e0c5299b90b863f375dade.png)
In the line of power of means, the harmonic mean is the mean of the -1 power.
Harmonic mean is commonly used to find the average of rates.
