Given that a, b, c, d are non-zero integers such that a < b < c < d If the mean and median of a, b, c, d are equal to zero, then a =____________ and b = ____________ .
If the median of the observations $x_1, x_2, x_3, x_4, x_5, x_6, x_7, x_8$ is m, then the median of the observations $x_3, x_4, x_5, x_6$ (where $x_1< x_2< x_3< x_4< x_5< x_6< x_7< x_8$) is ____________ .