I.12 — If a cone is cut by a plane through its axis, and by another plane perpendicular to the base of the axial triangle, and if the diameter of the section, when produced, meets one side of the axial triangle beyond the vertex, then the section has the property that the square on a line drawn from the section to its diameter equals a rectangle applied to a straight line increased by a certain segment; and such a section is called a hyperbola.