Question 13
$ABCD$ is a square of side length 10 cm. The segment $BC$ is fixed. The segment $AD$ moves in the plane to the segment $A^{\prime} D^{\prime}$ so that the lengths $AB$, $DC$ and $AD$ do not change. What is the area, in cm², of the shaded region in the diagram when the segment $A^{\prime} D^{\prime}$ intersects the segment $CD$ at its midpoint $G$?
