∑i=lrai+∑i=l+1rai+∑i=l+2rai+......+∑i=rrai\sum_{i=l}^ra_i + \sum_{i=l+1}^ra_i+\sum_{i=l+2}^ra_i +......+ \sum_{i=r}^ra_i ∑i=lrai+∑i=l+1rai+∑i=l+2rai+......+∑i=rrai 怎么求
