Three of Archimedes extant works are devoted to plane geometry. they are measurement of a Circle, Quadrature of the parabola, and on spirals. it was in the first of these that Archimedes inaugurated the classical method of computing , which we have already described in section 4-8 . in the second work, which contains 24 propositions, it is show that area of a parabolic segment is four thirds that of the inscribed triangle having the some base and having its opposite vertex at the point where the tangent is parallel to the base. the summation of a convergent geometric series is involved. the third work contains 28 propositions devoted to properties of the curve today know as the spiral of Archimedes and which has r=k for a polar equation. in particular, the area enclosed by the curve and two radii vectors is found essentially as it would be today in a calculus exercise. there are allusions to many lost works on plane geometry by Archimedes, and there is reason to believe that some of the theorems of these works have been preserved in the Liber assumptorum, a collection that has reached us through the Arabians (see problem study 6.4) one Arabian writer claims that Archimedes was the discoverer of the celebrated formula, for the area of a triangle in terms of its three sides. this formula has hitherto been attributed to Heron of Alexandria.