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Online Geometry Problem 733: Triangle, Orthocenter, Altitude, Reflection in a line, Circumcircle, Concurrency. Level: High School, Honors Geometry, College, Mathematics Education

In a triangle, let a line pass through the orthocenter and intersect the three sides at distinct points. Reflect this line across each side of the triangle to create three new lines. Prove that these three reflected lines intersect at a single point, which lies on the circumcircle of the triangle.

 Triangle, Orthocenter, Reflection, Circumcircle, Concurrency
 

Problem Statement with details

Let ABC be a triangle with orthocenter H. A line L passing through H intersects sides BC, CA, and AB at points A1, B1, and C1, respectively. Lines A1A2, B1B2, and C1C2 are the reflections of line L in sides BC, CA, and AB, respectively. Prove that lines A1A2, B1B2, and C1C2 are concurrent at a point P that lies on the circumcircle of triangle ABC.

Key Concepts:

  • Orthocenter: The point where the altitudes of a triangle intersect.
  • Reflection: A transformation that flips a figure across a line.
  • Concurrency: The property of three or more lines intersecting at a single point.
  • Circumcircle: The circle that passes through all three vertices of a triangle.

Flyer of problem 733 Designed with iPad Apps

 Diagram of Concurrency of Reflected Lines in a Triangle: Exploring the Orthocenter and Circumcircle designid with iPad Apple Intelligence

Home | SearchGeometry | Problems | All Problems | Open Problems | Visual Index | 731-740 | Triangles | Altitude | Orthocenter | Perpendicular lines | Circumcircle | Congruence | Reflection | Email | Solution / comment | by Antonio Gutierrez