Gergonne point. From Wikipedia, the free encyclopedia. Redirect page. Jump to navigation Jump to search. Redirect to: Incircle and excircles of a. The point of intersection of the lines joining the vertices of a triangle to the points where the sides of the triangle opposite to the vertices touch. Gergonne Point Theorem, Proof using Ceva’s Theorem – Antonio Gutierrez. Triangle, inscribed circle, and Concurrent Cevians. Geometry problems involving .

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This time a GSP sketch can be examined with a corresponding table of values which demonstrate that the Gergonne Point H is between each segment that connects the vertices to gergonbe incenter’s points of tangencies.

Nobbs’ Points, Gergonne Line

Now that it has been shown that the point B is between the other two points on each segment and that the point is on all three segments at the same time, then it must be a point of concurrency for all three segments. Click here for a GSP sketch in which a randomly formed triangle can be examined by using the selection tool to move the vertices to change the shape.

The Gergonne Point, so named after the French mathematician Joseph Gergonne, is the point of concurrency which results from connecting the vertices of a triangle to the opposite points of tangency of the triangle’s incircle.

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This essay will prove the existence of this point for any triangle, explore its relationship to the Euler line, if any exist, and discuss the possible usefulness of this point.

Gergonne point

Assume that there are at least two points of intersection between the lines. This is a very informal way to illustrate concurrency – go to the next section for more rigor using Ceva’s Theorem. Notice the relationship of the triangles in the figure and the greater than or equal to 4 ratio. Can you find any other special qualities of the Gergonne Point? Thus D’ and D have to be the same point which contradicts the assumption that there could be two distinct points of concurrency.

These linear equations might be explored on software such as Algebra Xpressor. These can be shown similar through alternate interior angles and vertical angles which lead to AA Similarity see figure below.

Another approach to showing the existence of the Gergonne Point is to use GSP to create axes and a grid in order to examine the three linear equations that could be formed from making segments that join the vertices of each triangle to the points of concurrency of the incenter to each side. The following sketch shows the Euler Line with the Gergonne Point.


The Euler Line is the result of connecting the point of concurrency of the orthocenter, centroid, and circumcenter. Ceva’s Theorem proves concurrency for a point by examining similar triangles and certain ratios that equal 1 see picture below.

Most geometry students are familiar with the several points of concurrency and the steps necessary to construct such points.

These might include some of the following points of concurrency click for a GSP sketch illustration: Although there does not seem to be any special theories concerning the Gergonne point itself, the point can be examined in regard gergonme certain ratios of triangles created by the inscribed circle of the incenter.

A key question that might be raised by students is whether this point of concurrency occurs for any shape of triangle. Can you prove this for any triangle? A possible relationship to the Euler Line can now be explored fro the Gergonne Point. The point of intersection of the three lines should be the ordered pair solution to the system of equations which would indicate the existence of the Gergonne Point.