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61= x2+y2=8x即r=8c0s, 4 y=2x即a2= arctan2 D 8 cose 4 cose 62 4 arctan 2 acos de. f(rose, sine)rdr cost0 y x 4 2 2 x + y = x 8 2 2 x + y = x x = y   =  即  y = 2x 即   = arctan D r = 8 cos 4 8 . r = 4 cos  2 1 x y, y x所 围 D : x y x , x y x, = =  + =  + =      即 r = cos ,   arctan2 4 8cos 4cos π d ( cos , sin ) d θ θ θ f r θ r θ r r 即 r = cos , 23. .  = D 计算 I f (x, y)dxdy I =