Every lesson in this module is about an overlay of two intersecting lines and a circle. This will be pointed out to students later in the module, but keep this in mind while presenting the lessons.
In this lesson, students investigate what some say is the oldest recorded result, with proof, in the history of geometry— Thales’ theorem, attributed to Thales of Miletus (c.624–c.546 BCE), about 300 years before Euclid. Beginning with a simple experiment, students explore the converse of Thales’ theorem. This motivates the statement of Thales’ theorem, which students then prove using known properties of rectangles from Module 1.
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