Tuesday, July 22, 2008

Elearn Geometry Problem 138 Nagel's Theorem

See complete Problem 138
Nagel's Theorem, Orthic Triangle, Altitudes, Circumradius, Perpendicular. Level: High School, SAT Prep, College geometry

Post your solutions or ideas in the comments.


  1. as O is the circumcenter,
    as FACD concyclic,
    thus BO⊥DF is obvious, so are the others.

  2. angleBAO=angleABO=90-C and angleAHE=angleAFE(H is ortho centre and AFHE is cyclic) but angle AHE=90-(90-C)=C so angleAFE=C hence AO is perpendicular to EF simillarly remaining we can prove

  3. The acute angle between the tangent at B to the circumcircle and the chord BA equals the angle in the alternate segment angle BCA which in turn equals angle DFB (ACDF being a cyclic quadrilateral).
    Therefore FD is parallel to the tangent at B and hence perpendicular to the radius BO.

  4. About problem 138.
    In the solution by Vijay (March 20, 2010), I didn't understand how the fact that angleAFE=C leads to the conclusion that AO is perpendicular do EF.
    Can someone help me?

  5. Form 1:

    O and H are isogonal conjugate, so the sides of pedal triangle of H are orthogonal to the respective line AO, BO or CO and reciprocally.

    Form 2:

    We know that A, B and C are the excenters of tr DEF, so by the well known existence of Bevan's point, we get that perpendiculars through A, B and C to the respective side of tr DEF are concurrent.

    Form 3:

    tr DEF is orthologic to tr ABC by definition, so by the orthology theorem, tr ABC must be orthologic to tr DEF.