Euclid, [i]The Elements,[/i] Book I[br]Proposition III: To cut a line by a given length not on the same line.

The equilateral triangle can be constructed as a Regular Polyon on AC, or in the manner of Proposition I.[br][br]In Geogebra, given segments AB = a > CD = b, [br]Proposition 3 can be carried out in a single command:[br]Segment[A, Intersect[Circle[A, b], a]]