Suppose that a box has its faces parallel to the coordinate planes and the points $\left(x_{1}, y_{1}, z_{1}\right)$ and $\left(x_{2}, y_{2}, z_{2}\right)$ are endpoints of a diagonal.
(a) Find the coordinates of the remaining six corners.
(b) Show that the midpoint of the line segment joining
$$ \left(x_{1}, y_{1}, z_{1}\right) \text { and }\left(x_{2}, y_{2}, z_{2}\right) \text { is } $$
$\left(\frac{1}{2}\left(x_{1}+x_{2}\right), \frac{1}{2}\left(y_{1}+y_{2}\right), \frac{1}{2}\left(z_{1}+z_{2}\right)\right)$