1-12 ABCDC BBDCA AC
13. 14. 15. 16.
17.解:
(1)
相减得:()
是首项为1,公差为1的等差数列
........................................................................5
(2),设其前项和为
则
........................................................................10
18.解
(1)
........................................................................6
(2)
........................................................................12
19.解:
(1)到的距离之和为
,