.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 have the same arithmetic AND geometric mean as the pair of elements , or what? And if this is so, why do you write and not ?
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?