Sumation rules

1/N * Sum(i,1,N)(Sum(k,1,D)(x[i][k] * w[k]) + b - t[i])`

现在的目的是将w[k]给提取出来

先用结合律将里面的加法分开=>

1/N * Sum(i,1,N)(Sum(k,1,D)(x[i][k] * w[k])) + Sum(i,1,N)(b - t[i])

然后用交换律交换两个sum的位置 =>

1/N * Sum(k,1,D)(Sum(i,1,N)(x[i][k] * w[k]))

然后提取 =>

1/N * Sum(k,1,D)(w[k] * Sum(i,1,N)(x[i][k]))

再将1/N放到最里面去 =>

Sum(k,1,D)(w[k] * Sum(i,1,N)(x[i][k] * 1/N))