Welcome to the Orthocenter of a triangle, Geometry Index. Site created by Antonio Gutierrez

Altitude Index.
Theorems and Problems.

Triangle Orthocenter by Folding, Video .
Dynamic Geometry Software: Tabula by Numeracy Works.
Step-by-Step construction, Manipulation, and animation.

Euclid's Elements Book I, 23 Definitions. One-page visual illustration.
Euclid's Elements Book. Index

Triangle Centers.

Distances between Triangle Centers Index.

Nine-Point Center, Nine-Point Circle, Euler Line (English version).
Interactive illustration.

Puzzle: Euler Line and Nine Point Center

Feuerbach Points and Nine-Point Circle with interactive animation, manipulation, and step-by-step construction.

Feuerbach Point Facts
Feuerbach Index

Feuerbach Theorem, Acute Triangle illustration
 

Feuerbach Theorem, Obtuse Triangle illustration
 

Feuerbach Theorem, Right Triangle illustration
 

Schiffler Point: Four Euler Lines with interactive animation and manipulation. Interactive Geometry Software.

Complete Quadrilateral: Ortholine-Steiner Line.
The orthocenters of the four triangles (BCE, CDF, ABF, ADE) of a complete quadrilateral ABCDEF lie on a line called Ortholine - Steiner Line.
Using Dynamic Software
Step-by-Step construction, Manipulation, and animation

Geometry Problem 721.
Triangle, Orthocenter, Circumcircle, Altitude, Collinear points.

Geometry Problem 715.
Triangle, Altitudes, Orthocenter, Parallel, Circumcircle, Angle, Measurement.

Geometry Problem 704.
Triangle, Orthocenter, Altitude, Midpoint, Perpendicular.

Geometry Problem 670.
Triangle, 60 Degrees, Orthocenter, Incenter, Circumcenter, Congruence.

Geometry Problem 664.
Triangle, 60 Degrees, Orthocenter, Incenter, Circumcenter, Angles, Triangle centers.

Geometry Problem 662.
Triangle, Incenter, Circumcenter, Orthocenter, Circle, Angle, Measurement.

Proposed Problem 427.
Triangle, Two Altitudes, Square of a Side, Orthocenter.

Proposed Problem 413.
Cyclic Quadrilateral, Orthocenter, Parallelogram, Concurrency, Congruence.

Proposed Problem 408.
Cyclic quadrilateral, Perpendicular, Parallelogram, Congruence, Orthocenter.

Proposed Problem 393.
Triangle, Orthocenter, Circumcircles, Congruence, Collinear.

Proposed Problem 191. Triangle, Altitudes, Orthocenter, Squares, Areas.

Proposed Problem 79: Triangle. Similarity, Altitudes, Orthocenter, Incircles, Inradii.

Proposed Problem 139. Triangle Area, Orthic Triangle, Semiperimeter, Circumradius.

Proposed Problem 138. Nagel's Theorem, Orthic Triangle, Altitudes, Circumradius, Perpendicular.

Proposed Problem 137. Orthic Triangle, Altitudes, Perpendicular, Incircle, Collinear Points, Parallelogram.

Proposed Problem 136. Orthic Triangle, Altitudes, Perpendicular, Concyclic Points.

Proposed Problem 135. Orthic Triangle, Altitudes, Perpendicular, Parallel

Proposed Problem 134. Orthic Triangle, Altitudes, Angle Bisectors, Orthocenter, Incenter.

Proposed Problem 132. Triangle, 60 degree, Orthocenter, Congruence, Midpoint.

Problem 37. Right triangle, altitude, incircles, incenters, and orthocenter.

Problem 21. Acute triangle, orthocenter, diameter, tangents.

Centroid /Barycenter. Index.

Journey to the Center of a Triangle (1976). Incenter, Circumcenter, Centroid, Orthocenter.

Geometry Online Glossary .
Geometry Glossary based on the new New York State mathematics standards initiative.

Right Triangle: Centroid. Centers, Inradius, Circumradius, Incenter, Circumcenter, Orthocenter, Centroid, Poncelet's theorem, Sagitta