\begin{equation}
\begin{array}{l}{\text { (a) Prove that the midpoint of the line segment from }} \\ {P_{1}\left(x_{1}, y_{1}, z_{1}\right) \text { to } P_{2}\left(x_{2}, y_{2}, z_{2}\right) \text { is }}\end{array}
\end{equation}
\begin{equation}
\left(\frac{x_{1}+x_{2}}{2}, \frac{y_{1}+y_{2}}{2}, \frac{z_{1}+z_{2}}{2}\right)
\end{equation}
\begin{equation}
\begin{array}{l}{\text { (b) Find the lengths of the medians of the triangle with ver- }} \\ {\text { tices } A(1,2,3), B(-2,0,5), \text { and } C(4,1,5) \text { . ( A median of a }} \\ {\text { triangle is a line segment that joins a vertex to the midpoint }} \\ {\text { of the opposite side.) }}\end{array}
\end{equation}