[ujikoluk]

That seems to show that there exist a and b such that the equality holds. But not that it holds for all a and b.

[wrsh07]

Which constraints on a,b (besides positive) does this proof require?

[xigoi]

a and b have to be real numbers, whereas the identity works for any commutative ring.

[phoe-krk]

> (besides positive)

You can chart a and b on a 2D coordinate system, where they're allowed to be negative. Even positivity is not strictly required here.

[davrosthedalek]

b<a Edit: and b,a element of R, but ok...

[notorandit]

Really? It even holds true for either a=0 or b=0.

[Scarblac]

But not for b > a.