Circle Theorems 1 - 2 Theorems

This applet investigates two important theorems about circles, angles in cyclic quadrilaterals and angles in the same segment.

Author and programmer: Ron Barrow

UK Years 10-11, Ages 14-16, KS4, Higher GCSE Mathematics - Geometry, Shape and Space

   
           

Instructions below   Waldomaths video   Waldomaths video See also:    2 More Circle Theorems   Alternate Segment Theorem   Circles and Tangents   Circles and Isosceles Triangles   Circles - Intersecting Chords   Circles - Radii and Chords   Parts of Circles PRINT

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How to Use this Applet

You can move the points A, B, C and D by clicking them and dragging them with your mouse. By doing this you'll see two important rules about angles inside circles being enacted. Look carefully at the size of the angles. Try to work out the rule before you click the button "Show Theorem" at the bottom. Clicking either of the boxes with "Show Chords" will draw chords which may help you with one of the theorems. Try it. You will flip from one rule to the other as the lines on the diagram cross over each other. Enjoy yourself while you practise and learn these two important theorems.