How do the axes of a coordinate system relate to each other?
See solution.
Practice makes perfect
Examining the square, we see that two opposite vertices are on the x-axis and the second pair of opposite vertices are on the y-axis. Since the axis of a coordinate plane are by definition perpendicular, the square's diagonals will be perpendicular as well.
General case
No matter the side of a square, when it's placed like above, one diagonal will run along the x-axis and the second will run along the y-axis.
Therefore, we can be sure that the two diagonals of a square are always perpendicular.
