⑩天掌 Teaching Plan on Advanced Mathematics o 例如,在平面上 {(x,y)x+y>0} 开区域 x {(x,y)1<x2+y2<4} 4{(x,y)x+y≥0} 闭区域 {(x,y)1≤x2+y2≤4} 2x x x tianjin polytechnic dmivendityTianjin Polytechnic University Teaching Plan on Advanced Mathematics 例如，在平面上 (x, y) x + y  0 ( , ) 1 4 2 2 x y  x + y  (x, y) x + y  0 ( , ) 1 4 2 2 x y  x + y  开区域 闭区域     x y o 1 2 x y o x y o x y o 1 2