contraharmonic mean

English edit

The contraharmonic mean of two variables is at the same distance from their arithmetic mean as their harmonic mean but on the opposite side.

contraharmonic mean (plural contraharmonic means)

(mathematics) A type of average calculated as the arithmetic mean of the squares of the values divided by the arithmetic mean of the values, ie. $C={{(x_{1}^{2}+x_{2}^{2}+...+x_{n}^{2}) \over n} \over {(x_{1}+x_{2}+...+x_{n}) \over n}}\ or\ C(x_{1},x_{2},...,x_{n})={{x_{1}^{2}+x_{2}^{2}+...+x_{n}^{2}} \over {x_{1}+x_{2}+...+x_{n}}}$
The contraharmonic mean of two numbers $a$ and $c$ is the number $b$ such that ${a-b \over c}={b-c \over a}$ . Combining the two ratios: ${b-c \over a}={(a-b)+(b-c) \over c+a}={a-c \over a+c}$ . Rotating the proportion 90° clockwise: ${a \over a+c}={b-c \over a-c}$ ; transposing twice to isolate the $b$ : $b=c+a\left({a-c \over a+c}\right)$ . Likewise $b=a-c\left({a-c \over a+c}\right)$ ; these are both equivalent to the more symmetrical $b={a^{2}+c^{2} \over a+c}$ .