Thomas' Calculus 13th Edition

a. Domain : The is set of all $(x,y)$ when $y -x \geq 0 \implies y \geq x$. b. Range is given as: $z\geq 0$ c. Level Curves: We have for $f(x,y)=0 \rightarrow$ origin and for $f(x,y)=c$; ellipses with center (0,0) and major and minor axes along $xy$-axes. d. No Boundary points e. Both Open and Closed f. Unbounded