关于本题式子的疑惑
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关于本题式子的疑惑
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封禁用户楼主2022/11/29 21:08
P[1,r]=∑lr12Nai(Nai−1)12(r−l+1)(r−l)=∑lr12(Nai2−Nai)12(r−l+1)(r−l)=12∑lrNai2−∑lrNai12(r−l+1)(r−l)=∑lrNai2−(r−l+1)(r−l+1)(r−l)P_{[1,r]} = \frac{\sum\limits^{r}_{l}\frac12N_{a_i}(N_{a_i} - 1)}{\frac12(r - l + 1)(r - l)} \\ = \frac{\sum\limits^{r}_{l}\frac12(N_{a_i}^2 -N_{a_i})}{\frac12(r - l + 1)(r - l)} \\ = \frac{\frac12\sum\limits^{r}_{l}N_{a_i}^2 -\sum\limits^{r}_{l}N_{a_i}}{\frac12(r - l + 1)(r - l)} \\ = \frac{\sum\limits^{r}_{l}N_{a_i}^2 - (r - l + 1)}{(r - l + 1)(r - l)} \\

为什么是这样的

2022/11/29 21:08