Count the Orthocenters

It's a common knowledge that three altitudes in a triangle intersect in a point known as the orthocenter of the triangle. So let's start with a triangle ABC and draw all three of its altitudes AH_a, BH_b, and CH_c.

The answer is 4 and, besides ABC, there are three more triangles: AHB, BHC, AHC, H being the orthocenter of ABC. Indeed, since AH_a is orthogonal to BC so BC is orthogonal to AH. (In general terms, the relation of orthogonality is symmetric and instead of claiming that one line is orthogonal to another we may simply say that the two lines are orthogonal.) Again, by the symmetry of orthogonality, AC serves as another altitude in ΔAHB. Thus, in ΔAHB we have three altitudes: CH_c, BC, and AC that intersect at the point C, the orthocenter of ΔAHB.

Similar considerations apply to the triangles BHC and AHC.

References

1. D. Wells, You are a Mathematician, John Wiley & Sons, 1995

The Orthocenter

1. Count the Orthocenters
2. Distance between the Orthocenter and Circumcenter
3. Circles through the Orthocenter
4. Reflections of the Orthocenter
5. CTK Wiki Math - Geometry - Reflections of the Orthocenter
6. Orthocenter and Three Equal Circles
7. A Proof of the Pythagorean theorem with Orthocenter and Right Isosceles Triangles
8. Reflections of a Line Through the Orthocenter
9. Equal Circles, Medial Triangle and Orthocenter
10. All About Altitudes
11. Orthocenters of Two Triangles Sharing Circumcenter and Base
12. Construction of a Triangle from Circumcenter, Orthocenter and Incenter
13. Reflections of the Orthocenter II
14. Circles On Cevians

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