\text { 28. } f(x, y)=\tan ^{-1}\left(\frac{y}{x}\right) In Exercises 17-30, (a) find the function's domain, (b) find the function's range, (c) describe the function's level curves, (d) find the boundary

of the function's domain, (e) determine if the domain is an open region, a closed region, or neither, and (f) decide if the domain is bounded or unbounded.