如果生产两种产量为z和y的商品的总成本函数为C(x，y)=15+2x2+xy+5y2(元),试求边际成本函数，并求x=3、y=6单位时的边际成本，说明其意义.

<br>由总成本函数<img src="/s/tiw/p3/UpLoadImage/2013-05-07/7135154a-7f6e-4b14-a301-3b4e8f71eabe.jpg" width="269" height="42">,则<br>两边对<img src="/s/tiw/p3/UpLoadImage/2013-05-07/a919bc47-4d42-4a5b-b3b0-1a55d777675f.png" width="13" height="15">求导得,关于<img src="/s/tiw/p3/UpLoadImage/2013-05-07/a919bc47-4d42-4a5b-b3b0-1a55d777675f.png" width="13" height="15">的边际成本函数为<img src="/s/tiw/p3/UpLoadImage/2013-05-07/11a22468-5853-462f-9000-5f589be9fe54.jpg" width="142" height="52"><br>两边对<img src="/s/tiw/p3/UpLoadImage/2013-05-07/0031d468-1ace-42ab-a474-22d261564c66.png" width="15" height="18">求导得,关于<img src="/s/tiw/p3/UpLoadImage/2013-05-07/0031d468-1ace-42ab-a474-22d261564c66.png" width="15" height="18">的边际成本函数为<img src="/s/tiw/p3/UpLoadImage/2013-05-07/fd2740ec-29a4-43ab-81a7-40a401a4b722.jpg" width="174" height="53"><br>当x=3,y=6时,边际成本为<img src="/s/tiw/p3/UpLoadImage/2013-05-07/43a6e34c-ef96-424e-9d75-613a5b1289b2.jpg" width="553" height="54"><br>前者的意义是:若<img src="/s/tiw/p3/UpLoadImage/2013-05-07/0031d468-1ace-42ab-a474-22d261564c66.png" width="15" height="18">保持在6单位不变,<img src="/s/tiw/p3/UpLoadImage/2013-05-07/a919bc47-4d42-4a5b-b3b0-1a55d777675f.png" width="13" height="15">在3单位的基础上再多生产一个单位产品,则成本增加18元.<br>后者的意义是:若<img src="/s/tiw/p3/UpLoadImage/2013-05-07/8e0b1174-302e-40e3-90ea-2b63b83a0ecb.png" width="13" height="13">保持在3单位不变,<img src="/s/tiw/p3/UpLoadImage/2013-05-07/0031d468-1ace-42ab-a474-22d261564c66.png" width="15" height="18">在6单位的基础上再多生产一个单位产品,则成本增加63元.