Calculate angles where circles are involved

Circle Theorems

1. An angle subtended at the centre, is twice the angle subtended at edge

2. The angle at the edge of semi circle, subtended by the diameter, is always a right angle

3. Opposite angles in a quadrilateral, where all its corners are on the edge of a circle, sum to 180^o So in the below example D^o = 78^o and C^o = 63^o

4. Angles subtended on the same arc are equal

You will need to remember these theorems and be able to recognise them in order to solve problems.

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