Geometry Problem 1129: Isosceles Triangle, Circumcenter, Incenter, Parallel Lines, Perpendicular Lines. Level: High School, SAT Prep, Honors Geometry, College, Math Education

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The figure below shows an isosceles triangle ABC (AB = BC) with circumcenter O and incenter I. If ID is parallel to AB (D on BC), prove that DO and CI are perpendicular.

Infographic Geometry problem 1129 Isosceles Triangle, Circumcenter, Incenter, Parallel Lines, Perpendicular Lines