1. 根据二重积分的几何意义求下列定积分: (1)
解(1)根据二重积分的几何意义,
??DR2?x2?y2d?,其中D?(x,y)x2?y2?R2;
????DR2?x2?y2d?,
D?(x,y)x2?y2?R2表示球心在坐标原点,半径为R的上半球的体
积,所以 习题8-2
1. 在直角坐标系下,将二重积分
????D2R2?x2?y2d???R3.
3??f(x,y)d?化为两种不同次序的二次
D积分,其中积分区域分别为: (1)D由y?x2与y?1围成的闭区域;
(2)D由y?sinx与y?0(0?x??)围成的闭区域; 解(1)(2)
?1?1dx?2f(x,y)dy,?dy?x011y?yf(x,y)dx;
??0dx?sinx0f(x,y)dy,?dy?01??arcsinyarcsinyf(x,y)dx;
2. 计算下列二重积分: (1)(2)(3)
??eDx?ydxdy, 其中D是由x?0,x?2,y?0, y?1所围成的矩形;
??xyd?,其中D是由直线y?1,x?2及y?x所围成的闭区域;
D22(x?y)d?,其中D是由直线y?x,y?2x及x?1所围成的闭??D区域; (4)域;
解(1) 因为D是矩形区域,且ex?y?ex?ey,所以
??xyd?,其中D是由抛物线yD2?x及直线y?x?2所围成的闭区
??eDx?y21x???dxdy???edx???eydy?? ?0??0?2232?(ex0)(ey10)?(e?1)(e?1)?e?e?e?1
(2)
??xyd?,其中D是由直线y?1,x?2及y?x所围成的闭 区域.
D2x解一 如图,将积分区域视为X—型,
??xyd?????D11xydydx???21?y2?x?x??1dx ?2?y2y?x??21?x3x??x4x2?21?dx???1. ????122848????y1解二 将积分区域视为Y—型,
o2??Dxyd?????21212y2?x?xydxdy???y??2ydy 1?2??1x2x??(3)
??2y4?y3??2y??dy??y??2?8???212x211?1. 8??(xD22?y)d???dx?(x2?y2)dy
0xy3??(xy?)0312xx4x31dx???dx??.
0331(4)D既是X—型,也是Y—型.但易见选择前者计算较麻烦,需
将积分区域划分为两部分来计算,故选择后者.
??xyd???D2?1??y?2y2?x2xydxdy????1?22??y??y?2y2dy
y2?125(y(y?2)?y)dy 2??12yo2?1y2?xD4y?x?21?y443y6?2???y?2y??2?436?5?5. 8x?1
但易见选择前者计算较麻烦,需将积分区域划分为两部分来计算,故选择后者.
3. 改变积分次序: 2(1)?1dy?y00f(x,y)dx;
(3)
?elnx1dx?0f(x,y)dy;
解(1)题设二次积分的积分限:??0?y?1?0?x?y2
可改写为???0?x?1??x?y?1,
即
?1y2110dy?0f(x,y)dx??0dx?xf(x,y)dy.
(3)题设二次积分的积分限:??1?x?e?0?y?lnx
可改写为??0?y?1?ey?x?e;
即
?edx?lnx10f(x,y)dy??1dy?e0eyf(x,y)dx
总复习题8
1. 填空题:
(1)设D:a?x?b,0?y?1,且
??yf(x)dxdy?1D?baf(x)dx? . (2)积分
(x?y)2dxdy? x???. y?1(3)设D:x??,y?1.则
??(x?siny)d?等于 . D,则