thales's theorem can also be used to find the centre of a circle using an object with a right angle, such as a set square or rectangular sheet of paper larger than the circle. the angle is placed anywhere on its circumference (figure 1). the intersections of the two sides with the circumference define a diameter (figure 2). repeating this with a different set of intersections yields another diameter (figure 3). the centre is at the intersection of the diameters.