假设求杨辉三角这一列
我们考虑这个格子：
然后对其不断展开
综上：
∑ i = 0 n ( i k ) = ( n + 1 k + 1 ) \sum_{i=0}^n\binom i k=\binom {n+1}{k+1} i=0∑n(ki)=(k+1n+1)
∑ i = l r ( i k ) = ( r + 1 k + 1 ) − ( l k + 1 ) \sum_{i=l}^r\binom i k=\binom{r+1}{k+1}-\binom{l}{k+1} i=l∑r(ki)=(k+1r+1)−(k+1l)
![[MySQL]BLOB/TEXT column ‘xxx‘ used in key specification without a key length](https://img-blog.csdnimg.cn/f10f57a02d3a48d5ad970bf30c52ab4c.png)
