Answer:
[tex]\begin{equation*} (\frac{a}{2},\frac{a}{2}) \end{equation*}[/tex]Explanation:
Given the coordinates as A(0,0), B (0,a), C(a,a), and D(a,0), let's go ahead and draw a sketch as seen below;
Recall the below midpoint formula;
[tex]Midpoint=(\frac{x_1+x_2}{2},\frac{y_1+y_2}{2})[/tex]Solving for the midpoints of AC, we'll have;
[tex]\begin{gathered} Midpoint=(\frac{0+a}{2},\frac{0+a}{2}) \\ \\ =(\frac{a}{2},\frac{a}{2}) \end{gathered}[/tex]