Consider any convex quadrilateral <i>Q</i> made from four rigid rods


with flexible joints at the vertices so that the shape of <i>Q</i> can


be changed while keeping the lengths of the sides constant. If the


diagonals of the quadrilateral cross at an angle &#952; in the


range (0  &#8804; &#952; &#x003C;  &#960;/2), as we deform <i>Q</i>, the angle


&#952; and the lengths of the diagonals will change. Using the


results of the two problems on quadrilaterals in the October 2002


15+ Challenges prove that the area of <i>Q</i> is proportional to


tan&#952;.