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Circle theorems – interactive GeoGebra applets Drag points around to see live illustrations of the circle theorems:
A triangle formed by two radii and a chord is isosceles
The angle at the centre is double the angle at the circumference
Note that this is true even if the angle at the centre is reflex and even if a chord intersects a radius.
The angle in a semicircle is a right angle
This is actually a special case of the theorem about the angle at the centre being double the angle at the circumference:
Angles in the same segment are equal
Opposite angles in a cyclic quadrilateral add up to 180º
are four-sided shapes whose corners all lie on the circumference of a circle.
Make sure that vertices A, B, C and D appear in that order as you go anticlockwise around the circle. Notice that if you drag the vertices so that the order is e.g. A, C, B, D, you will no longer have a quadrilateral.
The angle between a tangent and radius is 90º
Two tangents from a common point to two points on the circle are the same length
With thanks to Michael Borcherds, whose Common Tangents to a Circle applet is available here. Material modified and embedded here under the CC-BY-SA 3.0 license.
The perpendicular line from the centre of the circle to a chord bisects the chord
> With thanks to Adam Antonio, whose Chord Properties 1 applet is available here. Material modified and embedded here under the CC-BY-SA 3.0 license.
The angle between a tangent and chord is equal to the angle in the alternate segment
Circle theorems – exam-style questions
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Links to past exam questions
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Proofs of the circle theorems Video under construction
In the real world
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