Exploring Thales’ Theorem

I was playing with a geometry software package and decided to explore Thales Theorem.

The theorem states that for any diameter line drawn through the circle with endpoints B and C on the circle (obviously passing through the circle’s center point), any third non-collinear point A on the circle can be used to form a right angle triangle. That is, no matter where you place A on the circle, the angle BAC is always a right angle. Most places I have read online stop there.

There was one small problem on my software. Since constructing this circle meant that the center point was already defined on my program, there didn’t seem to be a way to make the center point part of the line, except by manipulating the mouse or arrow keys. So, as a result, my angle ended up being slightly off: 90.00550^{\circ} was the best I could do. But then, I noticed something else: No matter where point A was moved from then on, the angle would stay exactly the same, at 90.00550^{\circ}.

Now, 90.00550^{\circ} is not a right angle. Right angles have to be exactly 90^{\circ} or go home. If it’s not a right angle, then Thales’ theorem should work for any angle.

Why not restate the theorem for internal angles in the circle a little more generally then?

For any chord with endpoints BC in the circle, and a point A in the major arc of the circle, all angles \angle BAC will all equal some angle \theta. For points A in the minor arc, all angles will be equal to 180^{\circ} - \theta.

Note that BC is the desired chord, making the arc containing point A the major arc, with the small arc in the lower part of the circle the minor arc. As shown, all angles in the major arc are about 30.3 degrees.

So, now the limitations of my software are unimportant. In the setup shown on the left, the circle contains the chord BC, and A lies in the major arc, forming an angle \angle BAC = 30.29879^{\circ}. If A lay in the minor arc, the angle would have been 180^{\circ} - 30.29879^{\circ} = 149.70121^{\circ}.

By manipulating BC, you can obtain any angle \angle BAC you like, so long as \angle BAC < 180^{\circ}. More precisely, all angles in the minor arc drawn in the manner previously described will be 90^{\circ} < \angle BAC < 180^{\circ}, and all angles in the major arc will tend to be: 0^{\circ} < \angle BAC < 90^{\circ}. If the chord is actually the diameter line of the circle, then \angle BAC = 90^{\circ} exactly.

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I am Paul King, a math and science teacher. I help maintain the MIT FAQ Archive along with Nick Bolach. I am also the maintainer of the FAQ for sci.bio.food-science.