Chapter 12.1, Problem 21E

(a) Prove that the midpoint of the line segment from P₁(x₁, y₁, z₁) to P₂(x₂, y₂, z₂) is

$\left(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2},\frac{z_1+z_2}{2}\right)$

(b) Find the lengths of the medians of the triangle with vertices A(1, 2, 3), B(−2, 0, 5), and C(4, 1, 5).    (A median of a triangle is a line segment that joins a vertex    to the midpoint of the opposite side.)