Harmonic mean
The harmonic mean (frequently referred to as HM) is a special kind of mean (like Arithmetic mean, Geometric mean). The harmonic mean of n numbers 
is defined to be: 
.
The harmonic mean is a part of a frequently used inequality, the Arithmetic mean-Harmonic mean-Geometric mean inequality. The Inequality states that for a set of positive numbers 
: ![$\frac{x_1+x_2+\ldots+x_n}{n}\ge \sqrt[n]{x_1\cdot x_2 \ldots x_n}\ge \frac{n} {\frac{1}{x_1}+\frac{1}{x_2}+...+\frac{1}{x_n}}$](http://latex.artofproblemsolving.com/3/9/e/39ec77ffe648183e4dd5abe7e69844e45fd7c2db.png)
