Legality of posting some solutions of math problems from a textbook to the internet?

I know I should consult a lawyer, but I just want a preliminary opinion from Fluther.

So, I want to post some problems from a math textbook with my solutions, but would this be legal? I wouldn’t be profiting, it’d be for educational purposes, and of course, I’d acknowledge the source. I looked up “fair use” and from what I understand, what it boils down to is whether I’d be reducing the market for the original, which I most certainly wouldn’t…and I doubt that in practice I’d get in serious trouble (talk about bad publicity for the textbook publisher). Still, I want a second (or third) opinion on this before I post anything on the internet.