postulado 71, poligonos concavos ordenados 21
The length of the sides of the parallelogram circumscribed to a polygon concave ordered of the form only one of dato ( m, r, n ) m greater that r, r greater n, n greater or equal to one, is equal to m by the length of the diagonal lower or to r by the length of the diagonal greater of them parallelograms congruent in that is divided the polygon concave ordered.