Consider any convex quadrilateral Q made from four rigid rods with flexible joints at the vertices so that the shape of Q can be changed while keeping the lengths of the sides constant. If the diagonals of the quadrilateral cross at an angle θ in the range (0 ≤ θ < π/2), as we deform Q, the angle θ 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 Q is proportional to tanθ.