Originally Posted by

**rainer** I have three known values: a, b, c, and a fourth unknown value: D.

I know that (a, b) has the same arithmetic-geometric mean as (c, D).

What do you mean here? The pair of elements $\displaystyle a,b$ have the same arithmetic AND geometric mean as the pair of elements $\displaystyle c,D$ , or what? And if this is so, why do you write $\displaystyle (a,b)$ and not $\displaystyle \{a,b\}$ ?

Tonio

I.e.:

$\displaystyle M(a, b)=M(c, D)$

What is the quickest way to derive the unknown value D from this info? Do I just have to "eyeball" it iteratively, or is there a quicker more effective method?

Thanks.