from what i know, for any positive number a and b,

i need to know if it is true all the time, whether i could say that even when i don't know for sure that a and b is always positive.

put another way, if i come across a question and my answer is where m and n are unknowns and can be any number( even a negative number), do i leave my answer as ? would it be correct or wrong to to simplify to ?