Thales' theorem

An applet is used to explore Thales' theorem: An angle inscribed in a semicircle is a right angle.

Interactive Tutorial

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1 - Press the button above to start the applet.

2 - A and B are the endpoint of a diameter of a circle. You may drag any of the vertices A, B or C of the triangle. Drag C so that it is on the circle. Read the value of
angle C (top left) and round it to the nearest degree. Drag points A and B to other position and repeat. What is the value of angle C when vertex C is on the circle?

3 - According to Thales' theorem, when angle C is inscribed in a semi circle its value is 90 degrees.

More geometry references
Geometry Tutorials, Problems and Interactive Applets.