Problem 4. Write the coordinates of point $C$, which divides the segment $A B$ in the ratio $2: 1$, starting from point $A$, if $A(1 ;-1 ; 2)$ and $B(7 ;-4 ;-1)$.
Solution. Obviously, $\overline{A C}=\frac{2}{3} \overline{A B}=\frac{2}{3}(6 ; \quad-3$; $-3)=(4 ;-2 ;-2)$. Therefore, $C(1+4 ;-1-2 ; 2-2)$, i.e., $C(5 ;-3 ; 0)$.