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# How do I calculate angles where circles are involved?

By Jack Putt on the 11th of June, 2012

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In your General Level Exam you might have to deal with angles which are features of a circle.

## There are some important facts you have to remember -

A diameter is a line which passes through the centre of a circle.

A radius is a line from the centre of a circle to the circumference.

A tangent is a line which touches a circle at one point only.

The angle between a tangent and a radius is a right angle of 90°.

Any angle in a semicircle is a right angle of 90°.

By Pinaki Panda on the 16th of January, 2013

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In your General Level Exam you might have to deal with angles which are features of a circle.

## There are some important facts you have to remember -

A diameter is a line which passes through the centre of a circle.

A radius is a line from the centre of a circle to the circumference.

A tangent is a line which touches a circle at one point only.

The angle between a tangent and a radius is a right angle of 90°.

Any angle in a semicircle is a right angle of 90°.

By Pinaki Panda on the 16th of January, 2013

## 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 180o So in the below example Do = 78o and Co = 63o

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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