1111b?1?C1n?1?2?Cn?2?3?Cn?3?????(n?2)?Cn?(n?2)?(n?1)?Cn?(n?1)，

1111a?2?C11?3?C2?4?C3?????(n?1)?Cn?2?n?Cn?1?? 6分

?2?1?3?2?4?3?????(n?1)?(n?2)?n?(n?1)．

32223223222则a?C22+C3?C4?????Cn?C3+C3?C4?????Cn?C4+C4?????Cn?????Cn?1 2所以a?2C3n?1．?? 8分 又a?b?(1?2?3?????n)?C1n?1?从而b?1．?? 10分。

a2

n(n?1)3?(n?1)?3C3n?1，所以b?Cn?1． 2