Referring to the figure, Khayyam takes the line segment AB to have length b. A per-pendicular BC of length c is then drawn to AB . Next he constructs a parabola with vertex B,axis BF,and parameter b. in modern notation, The parabola has equation x2=by . Now,on BC as diameter,a semicircle is described . Its equation is .The semicircle will meet the parabola at a point D whose abscissa , or x-coordinate,provides a root of the given cubic.Geometrically the root is represented by the line segment BE,with E determined by dropping a perpendicular from D to BC.