23计算I=/(x,y)dD:x2+y2=4x,x2+y2=8x, x=y,y=2x所围 x2+y2=4x即r=4c0s6, x=y即B,=z 4 x2+y2=8x即r=8c0s, y=2x即62= arctan2 =8c0s6 r=4 cos0 40 y 4 x r = 4 cos x y, y x所 围 D : x y x , x y x, = =  + =  + =      4 2 2 x + y = x 8 2 2 x + y = x r = 8 cos 8 D 1  2 即 r = cos ,   =  即  即 r = cos , 即   = arctan 23.  = D 计算 I f (x, y)dxdy y = 2x x = y