rt，就是对于 1≤i,j≤n1\le i,j\le n1≤i,j≤n，有 ∑k=0i−1(k+(j−1)k)+∑k=0j−1(k+(i−1)k)=(i+ji)\sum\limits_{k=0}^{i-1}\dbinom{k+(j-1)}{k}+\sum\limits_{k=0}^{j-1}\dbinom{k+(i-1)}{k}=\dbinom{i+j}{i}k=0∑i−1(kk+(j−1))+k=0∑j−1(kk+(i−1))=(ii+j)。不太懂为什么，求解释。
