[zerohp]

I think they would assume that if they had watched the lectures. I can't recall a single one where the final answer wasn't reduced.

[etha]

If they truly aim to teach math and not, say, engineering, then they should not be teaching students to assume constraints that aren't given in the problem.

[dhimes]

It's also usually just a part of the course- like putting units on an answer in physics. You don't have to say so in every problem because you are taught from the start how to make your responses correct. I built a lot of content for a student math problem system and this is what the teacher wanted. 6/9 is an incorrect answer in elementary mathematics.

[aik]

I agree, and in a lot of cases I find that to be a sad truth instead of acceptable.