By definition, the x-axis and y-axis are perpendicular to each other.
See solution.
Practice makes perfect
By definition, we know that the x-axis is perpendicular to the y-axis. Let's call an arbitrary horizontal line h and an arbitrary vertical line v. Since any two horizontal lines are parallel, we know that h is parallel with the x-axis. Similarly, because any two vertical lines are parallel, we know that v is parallel with the y-axis. Therefore, by the Perpendicular Transversal Theorem, h is perpendicular to the x-axis and v is perpendicular to the y-axis. Therefore, h∥ v and thus any horizontal line is perpendicular to any vertical line.
