Constructing a Square with a Given Diagonal

Construction Algorithm
1 Construct a line AC as a given diagonal of the square.
2 Construct a line at C so that it makes an angle of 135° measured anti-clockwise with the diagonal AC.
3 Construct another line at A so that it makes an angle of 45° measured anti-clockwise with the diagonal AC.
4 Generate the intersection point between the two constructed lines in steps 2 and 3 at B.
5 Construct the image of B by reflection on AC to find the fourth vertex D of the square.
6 Join up the four points A, B, C and D to get the required square.
7 Switch to Move Mode. Pick a free element. Move it around to check your construction.

Construction Theorems

The two diagonals of the square are perpendicular to each other and equal in length.

Each diagonal of the square bisects two opposite angles.

Reflection preserves the distance between points.

Please enable Java for an interactive construction (with Cinderella).
Please enable Java for an interactive construction (with Cinderella).