Geometry: Orthocenter of a triangle, Theorems and Problems - Index

Altitude Index.
Theorems and Problems.

Orthic Triangle Index.

GeoGebra, Dynamic Geometry: Orthocenter of a Triangle.
HTML5 Animation for Tablets (iPad, Nexus..)

GeoGebra, Dynamic Geometry: Internal Feuerbach Point
HTML5 Animation for Tablets (iPad, Nexus..)

GeoGebra, Dynamic Geometry: Nine Point Circle
HTML5 Animation for Tablets (iPad, Nexus..)

GeoGebra, Dynamic Geometry: Euler Line of a Triangle.
HTML5 Animation for Tablets (iPad, Nexus..)

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 837
Triangle, Orthocenter, Circumcenter, Circle, Concyclic Points.

Geometry Problem 784
Triangle, Orthocenter, Midpoint of a side, Perpendicular.

Geometry Problem 783
Triangle, Orthocenter, Circumcenter, Circumcircle, Diameter, Altitude, Midpoint.

Geometry Problem 782
Triangle, Orthocenter, Circumcenter, Circumcircle, Diameter, Altitude, Parallelogram.

Geometry Problem 781
Triangle, Orthocenter, Circumcenter, Vertex, Altitude, Midpoint, Perpendicular.

Geometry Problem 780
Triangle, Sum of Inradius and Circumradius, Distances from the Orthocenter to the Vertices.

Geometry Problem 779
Triangle, Altitude, Orthocenter, Midpoint, Angle, 90 Degrees.

Geometry Problem 778
Triangle, Distance from the Orthocenter to a Vertex, Circle, Circumradius, Side, Square.

Geometry Problem 777
Triangle, Distance from the Orthocenter to a Vertex, Circle, Circumradius, Cosine.

Geometry Problem 769
Triangle, Circumcircle, Circumcenter, Orthocenter, Midpoint of arc, Parallel to chord, Congruence.

Geometry Problem 753.
Triangle, Altitudes, Orthocenter, Sum of ratios.

Geometry Problem 742.
Scalene Triangle, Orthocenter, Centroid, Circumcenter, Circumradius, Midpoint, Distance, Square, Metric Relations.

Geometry Problem 732.
Triangle, Altitude, Orthic Triangle, Orthocenter, Congruence, Parallelogram.

Geometry Problem 733.
Triangle, Orthocenter, Altitude, Reflection in a line, Circumcircle, Concurrency.

Geometry Problem 731.
Cyclic quadrilateral, Triangle, Orthocenter, Altitude, Perpendicular, Parallelogram.

Geometry Problem 730.
Triangle, Altitude, Orthocenter, Perpendicular, Concurrent Lines.

Geometry Problem 729.
Triangle, Altitude, Orthocenter, Circumcircle, Perpendicular, Midpoint.

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