The proof follows from SimsonProperty.html , where it is proved that lines AH, AI are parallel to the corresponding Simson lines S(D) and S(E) respectively. Then it is created the isosceles trapezium DEIH, where angle(EOD) is the central angle (2phi) and angle(IAH) is viewing arc(IH) = arc(DE) from point A on the circumference of (c).

See the file SimsonDiametral.html where is discussed the case of diametral D and E i.e. of phi being equal to a right angle.

SimsonDiametral.html

SimsonGeneral.html

SimsonGeneral2.html

SimsonProperty.html

Simson_3Lines.html

SteinerLine.html

ThreeDiameters.html

ThreeSimson.html

WallaceSimson.html

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