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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.

Home | Search | Geometry | Problems | All Problems | Open Problems | Visual Index | 10 Problems | 1121-1130 | Triangle | Isosceles Triangle | Circumcenter | Incenter | Perpendicular lines | Parallel lines | Email | Solution / comment.Last updated: Jul 7, 2015 by Antonio Gutierrez.