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Geometry Problem 39 with Solution: Triangle, Angle Bisector, Incircle, Cyclic Quadrilaterals. (15) Conclusions


A circle of center O inscribed in a triangle ABC is tangent to sides BC, AC, and AB at points D, E, and F, respectively. Lines AO and FD meet at H, and lines CO and DF meet at G. Prove the following:

  1. AGFO is a cyclic quadrilateral

  2. CHDO is a cyclic quadrilateral

  3. AGFE  is a cyclic quadrilateral

  4. CHDE  is a cyclic quadrilateral

  5. Angles FAG, FEG and FOG are congruent

  6. Angles DCH, DEH and DOH are congruent

  7. AG is perpendicular to CG and CH is perpendicular to AH

  8. AG is parallel to ED and CH is parallel  to EF

  9. Angles BAC and EGH are congruent and angles ACB and EHG are congruent

  10. Angles ABC and GEH are congruent

  11. Triangles ABC and GEH are similar

  12. AGHC is a cyclic quadrilateral

  13. Angles CAH, EGO, EFO, and CGH are congruent

  14. Angles ACG, EHO, EDO, and AHG are congruent

  15. O is the incenter of triangle EGH



See solution below

See also:
Help of problem 39
Art of problem 39

Sketch of problem 39

Home | Geometry | Problems | Open Problems | 10 Problems | Art | 31-40 | Circle Tangent Line | Right triangle, Altitude and Incircle | Email | by Antonio Gutierrez
Last updated: Sep 19, 2014

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Geometry Problem 39 Solution(s)