What has remained the same and what has changed with this reflection?
What has remained the same is the lengths of the sides of the triangle. Reflection has preserved the lengths of the sides of the triangles, meaning measurements like AD, D'M', and M'A' remain unchanged.
What has changed is the position of the triangle and the coordinates of its vertices. Because the triangle is reflected in the y-axis, the image shifts to the opposite side, and the signs of the x-coordinates of the vertices change (they become negative), altering the triangle's placement.
Explanation
The textbook context explicitly states that 'reflection has preserved the lengths of the sides of the triangles' after showing the distance calculations for the original and reflected triangles. It also demonstrates that the coordinates of the image points (like A', D', M') have negative x-coordinates compared to the original, indicating a change in position and coordinate values.