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Combine the result from the problem Flexi Quad Areas that the area $A(Q) = {\textstyle{1\over 2}}d_1d_2\sin\theta$ with the definition of the scalar product and use the result from the problem Flexi Quads that the scalar product of the diagonals is constant.
Flexi Quad Tan
Age 16 to 18
Challenge Level 
Combine the result from the problem Flexi Quad Areas that the area $A(Q) = {\textstyle{1\over 2}}d_1d_2\sin\theta$ with the definition of the scalar product and use the result from the problem Flexi Quads that the scalar product of the diagonals is constant.
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