22n?1n?1?n?1n?????2??3??3???3??3? （3）b?3a?14a?1?3?????4???3????????? ?????nnn?1?4???4???4???????4???????3?令b?y，u???n?4?n?122??111??????3， 则y?3u???3u??????????12分 ??????4242????392791*距最近， ，，，??，经比较?n?N，?u的值分别为141664162189∴当n?3时，bn有最小值是?，??????????????15分
256当n?1时，bn有最大值是0 ????????????????18分