Definition:Harmonic Mean

Let $$x_1, x_2, \ldots, x_n \in \mathbb{R}$$ be real numbers which are all positive.

The harmonic mean of $$x_1, x_2, \ldots, x_n$$ is defined as:

$$H_n^{-1} = \frac 1 n \left({\sum_{k=1}^n \frac 1 {x_k}}\right)$$

That is, to find out the harmonic mean of a set of $$n$$ numbers, take the reciprocal of the arithmetic mean of their reciprocals.