#### Answer

$\frac{x_1+x_2}{2}$;
$\frac{y_1+y_2}{2}$

#### Work Step by Step

RECALL:
The midpoint of the points $(x_1, y_1)$ and $x_2, y_2)$ is given by the formula:
$\text{midpoint} =\left(\frac{x_1+x_2}{2}, \frac{y_1+y_2}{2}\right).$
Thus, the missing expressions in the given statement are:
$\frac{x_1+x_2}{2}$;
$\frac{y_1+y_2}{2}$