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 SpaceTweet
Instructions below 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
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.