Answer
$(\frac{a}{2}, \frac{b}{2})$.
Work Step by Step
RECALL:
The midpoint $d$ of the points $(x_1, y_1)$ and $(x_2, y_2)$ can be found using the midpoint formula:
$\text{midpoint} = \left(\frac{x_1+x_2}{2}, \frac{y_1+y_2}{2}\right)$
Use the midpoint formula above to obtain:
$\text{midpoint}=\left(\frac{a+0}{2}, \frac{b+0}{2}\right)=\left(\frac{a}{2}, \frac{b}{2}\right)$
Thus, the midpoint of the line segment joining the two given points is $(\frac{a}{2}, \frac{b}{2})$.