Thales' Theorem Activities

Instructor: Clio Stearns

Clio has taught education courses at the college level and has a Ph.D. in curriculum and instruction.

Learning about Thales' Theorem is a significant part of understanding the geometry of a circle. This lesson offers activities that will make Thales' Theorem more accessible to your students.

Teaching Thales' Theorem

As a geometry teacher, you will be presenting your students with a wide range of different theorems including Thales' Theorem. This theorem states that if line AC is the diameter of a circle and point B is another point on the circle, the angle ABC will always be a right angle.

To teach students about Thales' Theorem, you can incorporate some activities into your instruction. The activities in this lesson will help students make meaning out of Thales' Theorem.

Visual Activities

This section offers activities that will appeal to visual learners as they come to understand Thales' Theorem.

Try to Prove it Wrong

One great way for students to see the efficacy of a theorem is to give them a chance to disprove it.

Working in partnerships or independently, ask students to try to draw the diameters of several different circles. Ask them to see if they can find a point on the circle at which angle ABC will not, in fact, be a right angle. Let them use protractors to measure the angles they form. Students will not be able to disprove the theorem, but they will learn significantly more about how and why it works.

Illustrate a Proof

This is another activity that students can do independently or with partners.

Let them look at the proof for Thales' Theorem and analyze it. Then, ask them to create a short comic strip or booklet that illustrates the proof one step at a time. Their illustrations should be accurate, but they can also be funny and creative. Students should show how and why the theorem is true. Finally, let students share what they came up with.

Kinesthetic Activities

These activities let students use hands and bodies to learn more about Thales' Theorem.

Act Out an Interview

Students might be intrigued that very little is actually known about Thales and his writing.

Break them into partnerships and let them learn what they can about Thales and the work that led up to his theorem. Then, have them act out a scene in which a reporter is interviewing Thales. Encourage them to get as dramatic as they can, using gestures and facial expressions to dramatize what they imagine Thales' discovery to have felt like.

Making Angles in Circles

Bring students out to the recess yard for this activity.

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