Two Convex Quadrilaterals and their Areas

Submitted by Isoscel on Mon, 11/26/2007 - 18:25.

Let be $A_1B_1C_1D_1$ a convex quadrilateral and A,B on $A_1B_1$ such as $A_1A=AB=BB_1$ and
C,D on $C_1D_1$ such as $C_1C=CD=DD_1$ Prove that