circletheorems-

7/28/2019 circletheorems-

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Circle Theorems


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A Circle features.

the distance around

the Circle

its PERIMETER

Diameter

the distance across

the circle, passingthrough the centre ofthe circle

Radius

the distance from

the centre of the circleto any point on thecircumference


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A Circle features

chord divides circle

into two segments

MajorSegment

MinorSegment

a line which touches the

circumference at one pointonly

From Italian tangere, to

touch


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Properties of circles

When angles, triangles and quadrilaterals

are constructed in a circle, the angles

have certain properties We are going to look at 4 such properties

before trying out some questions together


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An ANGLE on a chord

An angle that sits on a

chord does not change as

the APEX moves around

the circumference

as long as it stays

in the same segment

We say Anglessubtended by a chord

in the same segment

are equal

From now on, we will only consider the CHORD, not the ARC


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Typical examples

Find angles a and b

Imagine the ChordAngle b = 28

Imagine the Chord

Angle a = 44

Very often, the exam

tries to confuse you by

drawing in the chords

YOU have to see the

Angles on the same

chord for yourself


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Angle at the centre

Consider the two angles

which stand on this

same chord

What do you noticeabout the angle at the

circumference?

It is half the angle at the

centre

We say If two angles stand on the same chord,

then the angle at the centre is twice the angle at

the circumference

A


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Angle at the centre

We say If two angles stand on the same chord,

then the angle at the centre is twice the angle at

the circumference

Its still true when we move

The apex, A, around the

circumference

AAs long as it stays in the

same segment

136

272

Of course, the reflex angle

at the centre is twice the

angle at circumference too!!


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Angle at CentreA Special Case

When the angle stands

on the diameter, what is

the size of angle a?

aa

The diameter is a straight

line so the angle at the

centre is 180

Angle a = 90

We say The angle in a semi-circle is a Right Angle


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A Cyclic Quadrilateralis a Quadrilateral

whose vertices lie on the

circumference of a circle

Opposite angles in aCyclic Quadrilateral

Add up to 180

They are supplementary

We say

Opposite angles in a cyclic quadrilateral add up to 180


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Questions


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Could you define a rule for this situation?


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Tangents

When a tangent to a circle is drawn, the

angles inside & outside the circle have

several properties.


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1. Tangent & Radius

A tangent is perpendicular

to the radius of a circle


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2. Two tangents from a point outside circle

PA = PB

Tangents are equal

PO bisects angle APB

g

g


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3 Alternate Segment Theorem

The angle between a tangent

and a chord is equal to any

Angle in the alternate segment

Angle between tangent & chord

Alternate Segment

Angle in Alternate Segment

We say

The angle between a tangent and a chord is equal to any

Angle in the alternate (opposite) segment

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