Combine the result from the problem Flexi Quad Areas that the area

<math xmlns="http://www.w3.org/1998/Math/MathML">
<mrow><mi>A</mi><mo stretchy="false">(</mo><mi>Q</mi><mo stretchy="false">)</mo><mo>=</mo>
<mfrac><mrow><mn>1</mn></mrow>
<mrow><mn>2</mn></mrow>
</mfrac>

<msub><mrow><mi>d</mi></mrow><mrow><mn>1</mn></mrow>
</msub>

<msub><mrow><mi>d</mi></mrow><mrow><mn>2</mn></mrow>
</msub>
<mi>sin</mi><mi>&#x03B8;</mi></mrow></math> 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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