imotraining 2010 projective geometry alexander remorov projective geometry alexander remorov alexanderrem gmail com harmonic division given four collinear points a b c d we dene their cross ratio as a ...
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...Imotraining projective geometry alexander remorov alexanderrem gmail com harmonic division given four collinear points a b c d we dene their cross ratio as ca da cb db notethatthe lengths are directed whenthecross is equal to say that bundle particular case which occurs in many problems when on line this order let p be point not with the pencil made up of lines pa pb pc pd called lemma intersect l at respectively then proof wolog using sine law cpa cpb dpa dpb weget and angles sin gives trigonometric denition corresponding follows from therefore for any can its ok because corollary extremely useful same notation if so abc e f sides bc ab fe g i ad cf concur rent use ceva menelaus consider circle does depend it quadrilateral acbd nice thing about allows you pencils circles outside or tangents through intersecting cdatq q pad pdb similarly ac wecannowapplylemma wetakep andcon sider intersection since resulting intersec tion form hence har monic lie m midpoint cd am whenever see things li...