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 Diagonals for Area and Flexi Quads
prove that the area of Q is proportional to tanθ.