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circle theoremAB is a chord of the circle. C and D are points on the circumference. Use your mouse to change the size of the circle,or the positions of the centre, points A, B, C or D. Observe the angles at C and D.

However you alter the diagram, the angles at C and D will remain identical to each other. Thus...

The angles in the same segment are equal

Software/Applets used on this page

This page uses JSXGraph

JSXGraph is a cross-browser library for interactive geometry, function plotting, charting, and data visualization in a web browser. It is implemented completely in JavaScript, does not rely on any other library. It uses SVG and VML and is fully HTML5 compliant.



A straight line joining two points on a circle or other curve.


a conic curve with equation (x-a)²+(y-b)²=r²


A rule that connects one value in one set with one and only one value in another set.

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