思考题解答 0= arccos T N=acos 6 D 0≤ 2 0≤r≤acos0 arccos f(r, 0)d8. 6 =-arccos ricos思考题解答 o x y r = acos a D a r  = arccos a r  = −arccos , 0 cos : 2 2             − r a D ( , ) . arccos 0 arccos  − = a r a r a I dr f r  d