Geometry Proof : Product of area ofdiagonally opposite triangles in a quadrilateral

In a quadrilateral, product of area of diagonally opposite triangles is same. Can someone please help with a proof. A simple proof would be welcome. Links to proof are also fine.

Re: Geometry Proof : Product of area ofdiagonally opposite triangles in a quadrilater

The area of a triangle with two sides and with included angle is

Call four the four sublengths of the diagonals and use the above formula four times.

Remember that