1. The problem statement, all variables and given/known data Describe in words the surface whose equation is given by theta = pi/4 2. The attempt at a solution It is a fairly simple question, but I'm just trying to understand why this is considered a half-plane that exists for x that is greater than or equal to 0. There are no restriction on r or z. Given this, cannot r be negative? Wouldn't this make a full-plane and let it extend for x less than 0? I just don't quite see why it cannot have the point (-1, pi/4, 0), for example, which would be present for x less than 0. Any feedback is always appreciated!