Assignment 9


Page Bird

Pedal Triangles




1a. Let triangle ABC be any triangle. Then if P is any point in the plane, then the triangle formed by constructing perpendiculars to the sides of ABC (extended if necessary) locate three points R, S, and T that are the intersections. Triangle RST is the Pedal Triangle for Pedal Point P.



Click here for a GSP script tool for the construction of a pedal triangle.


When  the centroid or the incenter is the P point of the pedal triangle, the pedal triangle always lies within the given triangle ABC.  Here is one such example.


To see a GSP in which the centroid and the Pedal Point P are the same click here.  To see a GSP file in which the incenter and the Pedal Point P are the same click here.



There are several cases when the orthocenter is pedal point P. 

Case 1:  Acute triangle ABC


The pedal triangle RST lies within ABC as seen below









Case 2: ABC is a right triangle

When ABC is a right triangle then H or P becomes a vertex, and triangle RST collapses to a straight line as seen below.





Case three:  Triangle ABC is an obtuse triangle

When ABC is an obtuse triangle, the orthocenter is located outside triangle ABC and part of triangle RST is also outside of triangle ABC.  See below:







Click here to use a GSP file with the orthocenter constructed as the pedal point P.  Just drag on A, B, or C to change the triangle and location of pedal point P or H.



When pedal point P is the circumcenter, the pedal triangle always lies inside triangle ABC, regardless of where the pedal point P lies. 


Click here to check it out.


When P lies on a vertex, triangle RST collapses to the altitude of triangle ABC.




9. Find all conditions in which the three vertices of the Pedal triangle are colinear (that is, it is a degenerate triangle). This line segment is called the Simson Line.


The example above where P lies on the vertex, is one example of the Simson Line.


Anytime a vertex of pedal triangle RST becomes the same as a vertex of triangle ABC then the pedal triangle degenerates. 


The best way to see this is by dragging point P until the vertices line up for the triangles.


Click here to investigate on your own.


One of the most interesting parts of the assignment came in the later explorations:


10. Locate the midpoints of the sides of the Pedal Triangle. Construct a circle with center at the circumcenter of triangle ABC such that the radius is larger than the radius of the circumcircle. Trace the locus of the midpoints of the sides of the Pedal Triangle as the Pedal Point P is animated around the circle you have constructed. What are the three paths?

As you can see the path of the locus for the midpoints are ellipses.  I had a problem constructing this because I initially had triangle RST limited by triangle ABC, so I was only getting a partial picture of the ellipses.  Click here to see a GSP file of this.



11. Repeat where the path is the circumcircle?


As you can see the paths are still ellipses, just fatter than for a larger circle.


Click here to see a GSP file of this picture.



11a. Construct lines (not segments) on the sides of the Pedal triangle. Trace the lines as the Pedal point is moved along different paths.

Tracing the Simson line as the pedal point was animated along the circumcircle was a neat image:



The beauty of GSP is that it makes visible geometric relationships that we would otherwise, in most cases, be blind to.  Even the invisible is ordered, and that is why this activity is so cool!  I think any student would be interested in this investigation. 


My next investigation would start with the question, so what are the foci of the ellipses, which are the locus of the midpoints of the sides of triangle RST?