Geometry construction using a compass and straightedge
On this page we show how to construct (draw) a 90 degree angle with compass and straightedge or ruler. There are various ways to do this, but in this construction we use a property of
We create a circle where the
of the desired right angle is a point on a circle. Thales Theorem says that any
diameter of a circle
a right angle to any point on the circle.
This construction works by using Thales theorem. It creates a circle where the apex of the desired right angle is a point on a circle.
The image below is the final drawing above with the red items added.
The line segment BC is a diameter of the circle center D
BC is a straight line through the center.
Angle BAC has a measure of 90°.
The diameter of a circle always subtends an angle of 90° to any point (A) on the circle.
See Thales theorem.