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Taking Off on a Tangent

Time Required Short (2-5 days)
Prerequisites You should either currently be taking or have already completed a first course in geometry. You must understand the concept and method of a mathematical proof.
Material Availability Readily available
Cost Very Low (under $20)
Safety No issues


This is an interesting geometry project that goes back to the time of Archimedes, the famous Greek mathematician. You can combine this mathematical project with computer science and take this ancient problem into the twenty-first century with a dynamic diagram using the geometry applet.


Prove that AC = EF.
Figure 3 (applet or image): Prove that AC = EF.


Andrew Olson, Ph.D., Science Buddies
Professor David Joyce, for the Geometry Applet

Cite This Page

MLA Style

Science Buddies Staff. "Taking Off on a Tangent" Science Buddies. Science Buddies, 28 June 2014. Web. 24 Oct. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Math_p018.shtml>

APA Style

Science Buddies Staff. (2014, June 28). Taking Off on a Tangent. Retrieved October 24, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Math_p018.shtml

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Last edit date: 2014-06-28


Important: You will need the current version of Java installed on your computer for this project. If you do not have Java, some figures and images in this project may not display properly in your browser. To check for the current version of Java on your machine, visit Java.

Figure 1 below shows an arbelos. What is an arbelos? The arbelos is the white region in the figure, bounded by three semicircles. The diameters of the three semicircles are all on the same line segment, AB, and each semicircle is tangent to the other two. The arbelos has been studied by mathematicians since ancient times, and was named, apparently, for its resemblance to the shape of a round knife (called an arbelos) used by leatherworkers in ancient times.

Figure 1 (applet or image): The arbelos is the white region between the three semicircles.
Figure 1. The arbelos.

Notes on How to Manipulate the Diagram

The diagram is illustrated using the Geometry Applet (by kind permission of the author, see Bibliography). If you have any questions about the applet, send us an email at: scibuddy@sciencebuddies.org. With the help of the applet, you can manipulate the diagram by dragging points.

In order to take advantage of this applet, be sure that you have enabled Java on your browser. If you disable Java, or if your browser is not Java-capable, then an image may display stating "This plugin is disabled" or the diagram may appear— but as a plain, still image.

If you click on a point in the diagram, you can usually move it in some way. The free points, usually colored red, can be freely dragged about, and as they move, the rest of the diagram (except the other free points) will adjust appropriately. Sliding points, usually colored orange, can be dragged about like the free points, except their motion is limited to either a straight line, a circle, a plane, or a sphere, depending on the point. Other points can be dragged to translate the entire diagram. If a pivot point appears, usually colored green, then the diagram will be rotated and scaled around that pivot point. (Note that diagrams will often use only one or two of the above types of points.)

You can't drag a point off the diagram, but frequently parts of the diagram will be moved off as you drag other points around. If you type r or the space key while the cursor is over the diagram, then the diagram will be reset to its original configuration.

You can also lift the diagram off the page into a separate window. When you type u or return the diagram is moved to its own window. Typing d or return while the cursor is over the original window will return the diagram to the page. Note that you can resize the floating window to make the diagram larger.

An interesting property of the arbelos is the "twin circles" discovered by Archimedes (see Figure 2, another dynamic diagram). If you draw circles tangent to the line CD and inscribed within the arbelos, the circles will be congruent. Archimides proved this in his Book of Lemmas (proposition 5).

Figure 2 (applet or image): Archimedes's Twin Circles.
Figure 2. The "twin circles" of Archimedes.

This project centers around a curious property of the arbelos that involves the twin circles. The line segment EF in Figure 3 passes through the center of the right-hand twin circle. Its endpoint F is the point of tangency where the right-hand twin circle touches the smaller semicircle of the arbelos. The endpoint E is the intersection of the extension of CD and the line from F through the center of the right-hand twin circle. Curiously enough, line segment EF is congruent with line segment AC, the diameter of the other semicircle inside the largest semicircle of the arbelos. Can you prove that EF = AC?

Figure 3 (applet or image): Prove that AC = EF.
Figure 3. Prove that AC = EF.

Terms and Concepts

To do this project, you should do research that enables you to understand the following terms and concepts:

  • Right triangles
  • Similar triangles
  • The arbelos
  • Mathematical proof


This problem, along with some very interesting background material on the arbelos, is presented in the following reference:

For more information on the arbelos and Archimedes twin circles, see:

The Math Forum at Drexel University has some good advice on how to build a mathematical proof:

The Geometry Applet was kindly provided to Science Buddies by its author, Professor David Joyce, who wrote it for illustrating an amazing online edition of Euclid's Elements. Book IV, Proposition 5, and Book VI, Proposition 8 will be helpful:

Materials and Equipment

To do this experiment you will need the following materials and equipment:

  • Pencil
  • Paper
  • Compass
  • Straightedge

If you want to make your own dynamic figure to illustrate your proof using the Geometry Applet, you'll also need:

  • computer with Internet access, Web browser and a text editor (like Notepad).

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Experimental Procedure

  1. Do your background research.
  2. Organize your known facts.
  3. Spend some time thinking about the problem and you should be able to come up with the proof.

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I did this project I Did This Project! Please log in and let us know how things went.


For a more basic project, see:

For another project that uses geometric inversion in a circle, see:

If you'd like to try making your own dynamic diagram with the Geometry Applet, check out these Science Buddies links:

Share your story with Science Buddies!

I did this project I Did This Project! Please log in and let us know how things went.

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