Two Convex Quadrilaterals and their Areas

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

$$Area (ABCD)=\frac{1}{3}Area(A_1B_1C_1D_1)$$

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