20.92:曲面x2+y2=az(a>0)与z=2a-x2+y2所围区域 2 ∫(x,y,z) dodd rdrdo 2 f(rose, sine, z )dz. g 柱面坐标 联立{ 2=c a-I 2a 解得交线L L z 0 D∠0 J r≤ 囧θ r r f r θ r θ z z a r a r π a d d ( cos , sin , )d 2 0 2 0 2    − = r r θ f r θ r θ z z D a r a d d r ( cos , sin , )d 2 2   − = 0 x z y . L D     =  r a z D : . z = 2a − r r = az 2 . .    = − = z a r r az 2 2 联立 柱面坐标  + = (  )   :曲面 x y az a 与 z = a − x  + y  所围区域    = = z a r a 解得交线 L： a 2a 20. . I f (x, y,z)dxdydz   =