思考题 交换积分次序|=2my(or(a20 思考题解答 0≤r≤ a cose =df(,O)d0. 6= arccos r=acos D arccos8 思考题 交换积分次序: ( , ) ( 0). cos 0 2 2 =    − I d f r dr a a      思考题解答 , 0 cos 2 2 :       −       r a D ( , ) . arccos 0 arccos  − = a r a r a I dr f r  d o x y r = acos D a a r  = −arccos a r  = arccos