Question

Harmonic mean if a number h such that (h-a)/(b-h)=a/b Prove h is H(a,b) iff satisfies either relation a. (1/a)-(1/h) = (1/h) - (1/b) b. h = (2ab)/(a+b)

Answer 1

A.

$\displaystyle\frac1a-\frac1h=\frac1h-\frac1b$
$\implies\displaystyle\frac{h-a}{ah}=\frac{b-h}{bh}$
$\implies\displaystyle\frac{h-a}{b-h}=\frac{ah}{bh}$
$\implies\displaystyle\frac{h-a}{b-h}=\frac ab$

b.

$\displaystyle h=\frac{2ab}{a+b}$
$\displaystyle\implies\frac{h-a}{b-h}=\frac{\frac{2ab}{a+b}-a}{b-\frac{2ab}{a+b}}$
$\displaystyle\implies\frac{h-a}{b-h}=\frac{2ab-a(a+b)}{b(a+b)-2ab}$
$\displaystyle\implies\frac{h-a}{b-h}=\frac{ab-a^2}{b^2-ab}$
$\displaystyle\implies\frac{h-a}{b-h}=\frac{a(b-a)}{b(b-a)}$
$\displaystyle\implies\frac{h-a}{b-h}=\frac ab$

The other direction can be proved by following the manipulations in the reverse order.