Circle Theorems

1) Angles on a straight line add to 180^o

2) Base angles of an isosceles triangle are equal.

3) Angles in a quadrilateral add to 360^o

There are several key sentences that you must learn and know when to use each one.The sentences are:

1) The angle at the centre is double the angle at the circumference.

2) A triangle inside a semi circle is always right angled. (This can also be explained by sentence 1)

3) A tangent meets a radius at right angles.

4) Angles joined by the same arc are equal.

5) Opposite angles in a cyclic quadrilateral are equal.

6) Alternate angles are equal, alternate segment theory.

Quite often you will need to know other more basic angle facts and explain them correctly using the following sentences:

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