∑i=1n+m−1((n+m−1)×ai−∑i=1n+m−1aj)2n+m−1\frac{\sum_{i=1}^{n+m-1}((n+m-1) \times a_i - \sum_{i=1}^{n+m-1}a_j)^2}{n+m-1}n+m−1∑i=1n+m−1((n+m−1)×ai−∑i=1n+m−1aj)2
求化简的详细过程，悬 222 关
