Copyright © University of Cambridge. All rights reserved.

## 'Flexi Quad Tan' printed from http://nrich.maths.org/

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.