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 q in the
range (0 £ q < p/2), as we deform Q, the angle
q and the lengths of the diagonals will change.
prove that the area of Q is proportional to tanq.