a) \displaystyle \{(x,y)\in\mathbb{R}^2:\, 2x+y<4,\ x>0,\ y>0\}.
b) \displaystyle \{(x,y)\in\mathbb{R}^2:\, x^2+y^2<4,\ x < y,\ y>0\}
c) \displaystyle \{(x,y)\in\mathbb{R}^2:\, 2x+y<4,\ x+y>1\}.
