# 推导5：update b

已知：当$$0 \lt a\_i \lt C$$时，样本i位于间隔边界上，此时

$$
y\_i = \sum\_{j=1}^Na\_jy\_jK(i,j)+b
$$

1. 如果新算出来的a1或a2满足条件，任意一个代入公式（1）即可算出b。  
2. 如果a1和a2都满足条件，计算出来的b是相同的。  
3. 如果a1和a2都不满足条件，a1、a2分别代入公式（1）将得出两个不同的b，则取两个b的平均值。  


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