The Eyeball Theorem

Problem | Teachers' Notes | Hint | Solution | Printable page |
Stage: 4 and 5 Challenge Level: Challenge Level:3 Challenge Level:3 Challenge Level:3

Why do this problem?

In this problem, the dynamc geometry applet provides an opportunity for experimenting and making conjectures. The proof can be done entirely by similar triangles but there are several possible methods. A useful problem-solving skill to apply is to simplify the image and another is the magic of the key construction line that opens up new possibiliites.

Possible approach

Give the class time to experiment and make their own conjectures.

Make a list and chose one as a focus. The 'obvious' one is that the two chords are equal but they might, for example, notice that they are perpendicular to the line joining the centres.

Having identified a conjecture of interest, ask learners to write on separate cards statements about the figure that they know are true or think might be true (and distinguish between them).

In groups look at the the statements they have and try to arrange them into those they think are useful and those not and those aperson can justufy and those they cannot.
Now try to order or arrange them further discussing the possible use they might be or insights they might offer.
Can they come up with any further statements?

Share ideas ready to put together a more formal proof. Why not emphasisethe messiness of getting to a stage where enough is known, and a direction has emerged, before formalising a proof.

Key questions

  • Can you see any similar triangles?
  • Can you see symmetry in the diagram?
  • Is there a line you could draw that might give us a new insight?
  • Do we need all the diagram?

Possible extension

Try the problem Belt

Possible support

Encourage the learners to draw the dynamic diagram for themselves using Geogebra.


Published October 1998,September 2008.