Ah, yes I see. Good example. Thanks!

Wow, thank you very much Georgios. How can it be, that Maple does not automatically use expand first?

I got an answer (similar, maybe the same) by assuming w!=s, h!=-1, (1+r)*s!=0 (and maybe some more). These restrictions are placed on the function in the first place (from it's derivation). When I define a function, can I choose it's domain? Let's say i have the function:

u(c):=c^2, c>0.

How can I define it? Do I need to assume(c>0)? What if I refer to u(w-s) later, will Maple assume, that w-s>0? It doesn't seem so. :-(