Godfrey, C. (Charles), 1873-1924, Siddons, A. W. (Arthur Warry) joint author · Cambridge, University Press · leaf 159
is harmonic. * This method of proof may be remembered as follows: the range ondiagonal 1 is projected on to diagonal 2, and back again on to diagonal 1;using the two vertices that lie in diagonal 3. 140 COMPLETE QUADRILATERAL The above proof is of interest as bringing out the principleof duality.…
Geometry, Modern
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