The formula for the midpoint of a line segment:
[tex](\frac{x_1+x_2}{2},\frac{y_1+y_2}{2})[/tex]
where x₁, y₁ - the coordinates of one endpoint; x₂, y₂ - the coordinates of the other endpoint
[tex]\hbox{endpoint} - (5,1) \\
x_1=5 \\ y_1=1 \\ \\ \hbox{midpoint} - (9,3) \\ \Downarrow \\ \frac{5+x_2}{2}=9 \\ 5+x_2=18 \\ x_2=13 \\ \\ \frac{1+y_2}{2}=3 \\ 1+y_2=6 \\ y_2=5 \\ \\
\hbox{the other endpoint} - \boxed{(13,5)}[/tex]
