Two pairs of conjugate points {A, A'}, {B, B'} on the same line define an involution.
Where are the double points? Where can we place point C so that it coincides with its conjugate, C'?
(A construction without proof.)
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Construction of the Involution defined by {A, A'}, {B, B'}: [url]http://www.geogebratube.org/material/show/id/243899[/url]
