284. Given the vertices of a triangle $A(2 ; 0 ; 3), B(-5 ; 2 ; 1), C(3$; $2 ; 1)$. Find the length of its median, drawn from vertex $A$.
Solution. The coordinates of point $D$ (Fig. 34) will be found using formulas (4):

$$
x=\frac{-5+3}{2}=-1 ; y=\frac{2+2}{2}=2 ; \quad z=\frac{1+1}{2}=1
$$

The length of $A D$ will be found using formula (1):

$$
A D=\sqrt{3^{2}+(-2)^{2}+2^{2}}=\sqrt{17} \approx 4.12
$$