a) \displaystyle D_f=\{(x,y)\in\mathbb{R}^2:\, (x-1)^2+(y-2)^2<2\}.
b) \displaystyle D_f=\{(x,y)\in\mathbb{R}^2:\, -1\le x+2y\le 1\}.
c) \displaystyle D_f=\{(x,y,z)\in\mathbb{R}^3:\, xyz> 0\}.
