Letting C coincide with B we obtain the figure below, in which Z

Desargues theorem now asserts that Z is a double point of the involution defined on line (e) by the intersection points W

Note that the other fixed point Z' of the involution on (e) is the harmonic conjugate of Z with respect to W

The above property can be used to construct the conic passing through three points and two tangents. This is discussed in Conic3Pts2Tangents.html

CrossRatio0.html

DesarguesInvolution.html

DesarguesInvolution2.html

FourPtsAndTangent.html

FourPtsAndTangent2.html

HomographicRelation.html

HomographicRelationExample.html

InvolutionBasic.html

InvolutionBasic2.html

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