3rd prep

Unit 5 Angles and arcs in the circle

Lesson (1): Central angles and measuring arcs:

Important corollaries

Corollary (1)

In the same circle (or in congruent circles),

if the measures of arcs are equal, then the lengths of the

arcs are equal, and conversely.

Corollary (2)

In the same circle (or in congruent circles),

if the measures of arcs are equal, then

their chords are equal in length, and

conversely.

Corollary (3)

If two parallel chords are drawn in a circle, then the measure of

the two arcs between them are

equal.

Corollary (4)

If a chord is parallel to a tangent of a circle, then the

measures of the two arcs between them

are equal.

M C

B D A

* *

M

C

B

D

A

M C

B D A

M C

B D

A * *

M

C

B D

A

M

C B

D

A

• It is the angle whose vertex is the center of the circle and the two sides are radii in the circle.

Central angle:

• Is the measure of the central angle opposite to it.

• 𝑚 ∠ 𝐴𝑀𝐵 = 𝑚(𝐴𝐵)

Measure of the arc:

• The minor arc AB and is denoted by 𝐴𝐵

• The major arc ADB and is denoted by 𝐴𝐷𝐵

Note that:

•Measure of the simicircle = 180°

•Measure of a circle = 360°Remarks:

• is a part of a circle's circumference proportional to its measure.

• 𝑇ℎ𝑒 𝑎𝑟𝑐 𝑙𝑒𝑛𝑔𝑡ℎ =𝑚𝑒𝑎𝑠𝑢𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑎𝑟𝑐

360°× 2 𝜋 𝑟

Arc length:

3rd prep Example (1)

Example (2)

3rd prep Example (3)

1) ∵ 𝐴𝐵 ̅̅ ̅̅ ̅𝑖𝑠 𝑎 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 ∴ (3𝑥 + 5) + 4𝑥 = 180° ∴ 7𝑥 + 5 = 180° ∴ 7𝑥 = 175 ∴ 𝑥 = 25°

2) 𝑚 (𝐴𝐶) = 2 × 25° = 50°

3) 𝑚 (𝐴𝐷) = 4 × 25° = 100° 4) 𝑚 (𝐵𝐶) = 180° − 50° = 130°

5) 𝑚 (𝐶𝐴𝐷) = 50° + 150° = 200° 6) 𝑚 (𝐶𝐵𝐷) = 360° − 150° = 210°

7) 𝑚 (𝐴𝐶𝐷) = 360° − 100° = 260° 8) 𝑚 (𝐴𝐷𝐶) = 360° − 50° = 310°

Assignment:

Booklet pages 48 & 49

3rd prep

Lesson (2): relation between inscribed and central angles:

Given: In the circle 𝑀 : ∠𝐴𝐶𝐵 is an inscribed angle, ∠𝐴𝑀𝐵 is a central angle

R.T.P: 𝑚(∠𝐴𝐶𝐵) =1

2𝑚(∠𝐴𝑀𝐵)

Proof: The first case: If 𝑴 belongs to one of the sides of the inscribed angle 𝑨𝑪𝑩:

∵ ∠𝐴𝐶𝐵 is an exterior angle of ∆𝐴𝑀𝐶 ∴ 𝑚(∠𝐴𝑀𝐵) = 𝑚(∠𝐴) + 𝑚(∠𝐶) (1) , ∵ 𝑀𝐴 = 𝑀𝐶 (two radii lengths) ∴ 𝑚(∠𝐴) = 𝑚(∠𝐶) (2) From (1) and (2) we get:

𝑚(∠𝐴𝐶𝐵) =1

2𝑚(∠𝐴𝑀𝐵)

Const.:

The second case: If 𝑴 lies inside the inscribed angle 𝑨𝑪𝑩:

Draw 𝐶𝑀⃗⃗⃗⃗⃗⃗ to cut the circle at 𝐷

From the first case: ∴ 𝑚(∠𝐴𝐶𝐷) =1

2𝑚(∠𝐴𝑀𝐷) ,

𝑚(∠𝐵𝐶𝐷) =1

2𝑚(∠𝐵𝑀𝐷)

By adding:

∴ 𝑚(∠𝐴𝐶𝐷) + 𝑚(∠𝐵𝐶𝐷) =1

2𝑚(∠𝐴𝑀𝐷) +

1

2𝑚(∠𝐵𝑀𝐷)

∴ 𝑚(∠𝐴𝐶𝐵) =1

2𝑚(∠𝐴𝑀𝐵)

Const.:

The third case: If 𝑴 lies outside the inscribed angle 𝑨𝑪𝑩:

Draw 𝐶𝑀⃗⃗⃗⃗⃗⃗ to cut the circle at 𝐷

From the first case: ∴ 𝑚(∠𝐴𝐶𝐷) =1

2𝑚(∠𝐴𝑀𝐷)

𝑚(∠𝐵𝐶𝐷) =1

2𝑚(∠𝐵𝑀𝐷)

Subtracting:

∴ 𝑚(∠𝐴𝐶𝐷) − 𝑚(∠𝐵𝐶𝐷) =1

2𝑚(∠𝐴𝑀𝐷) −

1

2𝑚(∠𝐵𝑀𝐷)

∴ 𝑚(∠𝐴𝐶𝐵) =1

2𝑚(∠𝐴𝑀𝐵)

• It is the angle whose vertex lies on the circle and its sides contain two chords of the circle.

inscribed angle:

•The measure of the inscribed angle is half the measure of the central angle, subtended by the same arc.

Theorem (1)

•The measure of the central angle equals twice the measure of the inscribed angle, subtended by the same arc.

Remark

M A

C

B

3rd prep

Given: 𝐴𝐵̅̅ ̅̅ and 𝐶𝐷̅̅ ̅̅ are two chords in a circle

intersecting at 𝐸

R.T.P: 1) 𝑚(∠𝐴𝐸𝐶) =1

2[𝑚(𝐴𝐶) + 𝑚(𝐵𝐷 )]

2) 𝑚(∠𝐶𝐸𝐵) =1

2[𝑚(𝐵𝐶 ) + 𝑚(𝐴𝐷 )]

Construction: Draw 𝐵𝐶̅̅ ̅̅ and 𝐴𝐷̅̅ ̅̅

Proof: ∵ ∠𝐴𝐸𝐶 is an exterior angle of ∆𝐸𝐵𝐶

∴ 𝑚(∠𝐴𝐸𝐶) = 𝑚(∠𝐵) + 𝑚(∠𝐶)

∵ 𝑚(∠𝐵) =1

2𝑚(𝐴𝐶) , 𝑚(∠𝐶) =

1

2𝑚(𝐵𝐷 )

∴ 𝑚(∠𝐴𝐸𝐶) =1

2𝑚(𝐴𝐶 ) +

1

2𝑚(𝐵𝐷 )

=1

2[𝑚(𝐴𝐶 ) + 𝑚(𝐵𝐷 )]

Similarly, if we draw 𝐴𝐶̅̅ ̅̅ (or 𝐵𝐷̅̅ ̅̅ ), we can prove

that: 𝑚(∠𝐶𝐸𝐵) =1

2[𝑚(𝐵𝐶 ) + 𝑚(𝐴𝐷 )]

Corollary (1):

• The measure of an inscribed angle is half the measure of the subtended arc.

Corollary (2):

The inscribed angle in a

semicircle is a right angle.

• The measure of the arc equals twice the measure of the inscribed angle, subtended by this arc.

• The inscribed angle which is right angle is drawn in a semicircle.

• The inscribed angle which is subtended by an arc of measure less than the measure of a semicirlce is an acute angle.

• The inscribed angle which is subtended by an arc of measure greater than the measure of a semicirlce is an obtuse angle

Remarks

• If two chords intersect at a point inside a circle, then the measure of the included angle equals half of the sum of the two measures of the opposite arcs.

well known problem (1)

D A

C B

E

M A

C

B

3rd prep

Given: 𝐶𝐵⃗⃗⃗⃗ ⃗ ∩ 𝐸𝐷⃗⃗ ⃗⃗ ⃗ = {𝐴}

R.T.P: 1) 𝑚(∠𝐴) =1

2[𝑚(𝐶𝐸 ) − 𝑚(𝐵𝐷 )]

Construction: Draw 𝐶𝐷̅̅ ̅̅

Proof: ∵ ∠𝐶𝐷𝐸 is an exterior angle of ∆𝐴𝐷𝐶

∴ 𝑚(∠𝐶𝐷𝐸) = 𝑚(∠𝐴) + 𝑚(∠𝐶)

∴ 𝑚(∠𝐴) = 𝑚(∠𝐶𝐷𝐸) − 𝑚(∠𝐶)

∵ 𝑚(∠𝐶𝐷𝐸) =1

2𝑚(𝐶𝐸) , 𝑚(∠𝐶) =

1

2𝑚(𝐵𝐷)

∴ 𝑚(∠𝐴) =1

2𝑚(𝐶𝐸 ) −

1

2𝑚(𝐵𝐷 )

=1

2[𝑚(𝐶𝐸 ) − 𝑚(𝐵𝐷 )]

Example (1)

• If two rays carrying two chords in a circle are intersecting outside it, then the measure of their intersecting angle equals half the measure of the major arc subtracted from it half of the measure of the minor arc included by the two sides of this angle.

well known

problem (2)

D A

C

B

E

3rd prep Example (2)

Example (3)

Assignment:

Booklet pages 51, 52, 54 & 55

3rd prep

Lesson (3): inscribed angles subtended by the same arc:

Given: ∠𝐶 , ∠𝐷 and ∠𝐸 are inscribed angles

subtended by 𝐴𝐵

R.T.P: 𝑚(∠𝐶) = 𝑚(∠𝐷) = 𝑚(∠𝐸)

Proof: ∵ 𝑚(∠𝐶) =1

2𝑚(𝐴𝐵 )

, 𝑚(∠𝐷) =1

2𝑚(𝐴𝐵 )

, 𝑚(∠𝐸) =1

2𝑚(𝐴𝐵 )

∴ 𝑚(∠𝐶) = 𝑚(∠𝐷) = 𝑚(∠𝐸)

D

A C B

Y

* *

X

A B

* *

X

C D

* *

Y

A B

X

C D

Y

• In the same circle (or in any number of circles) the inscribed angles of equal measures subtend arcs of equal measures.

The converse of the previous corollary

is true also

• In the same circle, the measures of all inscribed angles subtended by the same arc are equal.

Theorem (2)

• In the same circle (or in any number of circles) the measures of the inscribed angles subtended by arcs of equal measures are equal.Corollary

D

A

C

B

E

* *

*

3rd prep Example (1)

Example (2)

Example (3)

Assignment:

Booklet pages 57, 58 & 59

3rd prep

Lesson (4): The cyclic quadrilateral and its properties:

Given: 𝐴𝐵𝐶𝐷 is a cyclic quadrilateral

R.T.P: 1) 𝑚(∠𝐴) + 𝑚(∠𝐶) = 180°

2) 𝑚(∠𝐵) + 𝑚(∠𝐷) = 180°

Proof: ∵ 𝑚(∠𝐴) =1

2𝑚(𝐵𝐶𝐷) and 𝑚(∠𝐶) =

1

2𝑚(𝐵𝐴𝐷)

∴ 𝑚(∠𝐴) + 𝑚(∠𝐶) =1

2[𝑚(𝐵𝐶𝐷) + 𝑚(𝐵𝐴𝐷)]

=1

2 the measure of the circle =

1

2× 360° = 180°

Similarly: 𝑚(∠𝐵) + 𝑚(∠𝐷) = 180°

A summary of the properties in of the cyclic quadrilateral:

Each two angles drawn on one of its sides as a base and on one side of this side are

equal in measure.

𝑚 ∠1 = 𝑚(∠2)

𝑚 ∠3 = 𝑚(∠4)

𝑚 ∠5 = 𝑚(∠6)

𝑚 ∠7 = 𝑚(∠8)

Each two opposite angles are supplementary " their

sum = 180° "

𝑚 ∠𝐴 + 𝑚 ∠𝐶 = 180°

𝑚 ∠𝐵 + 𝑚 ∠𝐷 = 180°

The measure exterior angle at a vertex of a C.Q. is equal

to the measure of the interior angle at the opposte

vertex.

𝑚 ∠𝐴𝐷𝐸 = 𝑚 ∠𝐵

• It is a quadrilateral figure whose four vertices belong to one circle.The cyclic

quadrilateral

•Each of the rectangle, the square and the isoscles trapezium are cyclic quadrilaterals.

•Each of the parallelogram, the rhombus and the trapezium that is not isoscles are not cyclic quadrilaterals.

Remark

• In a cyclic quadrilateral, each two opposite angles are supplementary.Theorem (3)

A

C B

D 2 1

3

4

5 6 7

8 A

C B

D

**

** E C D

B A

3rd prep Example (1)

Example (2)

Example (3)

3rd prep Example (4)

Example (5)

Assignment: Booklet pages 61, 62 & 64

3rd prep

Lesson (5): The relation between the tangents of the circle:

Given: 𝐴 is a point outside the circle 𝑀 , 𝐴𝐵̅̅ ̅̅ and 𝐴𝐶̅̅ ̅̅ are two

tangent-segments to the circle at 𝐵 and 𝐶 respectively

R.T.P: 𝐴𝐵 = 𝐴𝐶

Const.: Draw 𝑀𝐵̅̅ ̅̅̅ , 𝑀𝐶̅̅̅̅̅ , 𝑀𝐴̅̅̅̅̅

Proof: ∵ 𝐴𝐵⃡⃗⃗⃗ ⃗ is a tangent to the circle 𝑀

∴ 𝑚(∠𝐴𝐵𝑀) = 90°

∵ 𝐴𝐶⃡⃗⃗⃗ ⃗ is a tangent to the circle 𝑀

∴ 𝑚(∠𝐴𝐶𝑀) = 90°

In ∆∆ 𝐴𝐵𝑀 , 𝐴𝐶𝑀:

{

𝑀𝐵 = 𝑀𝐶 (𝑡ℎ𝑒 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡𝑤𝑜 𝑟𝑎𝑑𝑖𝑖)

𝐴𝑀̅̅̅̅̅ 𝑖𝑠 𝑎 𝑐𝑜𝑚𝑚𝑜𝑛 𝑠𝑖𝑑𝑒 𝑚(∠𝐴𝐵𝑀) = 𝑚(∠𝐴𝐶𝑀) = 90° (𝑝𝑟𝑜𝑣𝑒𝑑)

∴ ∆𝐴𝐵𝑀 ≡ ∆𝐴𝐶𝑀 ,

And we deduce that: 𝐴𝐵 = 𝐴𝐶

First: The two tangents drawn at the two ends of a diameter in a circle are parallel.

Second: The two tangents drawn at the two ends of a chord of a circle are intersecting.

•The two tangent-segments drawn to a circle from a point outside it are equal in length.

Theorem (4)

3rd prep

1) 𝐴𝐵 = 𝐴𝐶 (tangent segments)

2) 𝑀𝐵 = 𝑀𝐶 = 𝑟

3) 𝐵𝐸 = 𝐶𝐸 , 𝐴𝑀⃡⃗⃗⃗⃗⃗ ⊥ 𝐵𝐶̅̅ ̅̅

4) 𝑚(∠𝐴𝐵𝑀) = 𝑚(∠𝐴𝐶𝑀) = 90°

i.e. The figure 𝐴𝐵𝑀𝐶 is a cyclic quadrilateral.

5) 𝑚(∠𝐵𝐴𝑀) = 𝑚(∠𝐵𝐶𝑀) = 𝑚(∠𝐶𝐴𝑀) = 𝑚(∠𝐶𝐵𝑀)

6) 𝑚(∠𝐴𝑀𝐵) = 𝑚(∠𝐴𝐶𝐵) = 𝑚(∠𝐴𝑀𝐶) = 𝑚(∠𝐴𝐵𝐶)

Common tangents of two distant circles:

In the opposite figures: 𝑨𝑩 = 𝑪𝑫

Corollary (1):

•The straight line passing through the center of the circle and the intersection point of the two tangents is an axis of symmetry to the chord of tangency of those two tangents.

Corollary (2):

•The straight line passing through the center of the circle and the intersection point of the two tangents bisects the angle between these two tangents. It also bisects the angle between the two radii passing through the two points of tangency.

• 𝑚 ∠1 = 𝑚 ∠2

• 𝑚 ∠3 = 𝑚 ∠4

•The inscribed circle of a polygon is the circle which touches all of its sides internally.

Definition

Remarks on theorem (4) and its corollaries:

3rd prep The position of the two circles One inside

the other &

Concentric

Touching

internally Intersecting

Touching

externally Distant

The number of common tangents 0 1 2 3 4

Example (1)

Example (2)

3rd prep Example (3)

Assignment: Booklet pages 66 & 67

3rd prep

Lesson (6): Angles of Tangency:

⁰

• It is the angle whose vertex is the center of the circle and the two sides are radii in the circle.

Central angle:

• Is the measure of the central angle opposite to it.

• 𝑚 ∠ 𝐴𝑀𝐵 = 𝑚(𝐴𝐵)

Measure of the arc:

•The ninor arc AB and is denoted by 𝐴𝐵

•The major arc ACB and is denoted by 𝐴𝐶𝐵

Note that:

•Measure of the simicircle = 180°

•Measure of a circle = 360°Remarks:

• is a part of a circle's circumference proportional to its measure.

• 𝑇ℎ𝑒 𝑎𝑟𝑐 𝑙𝑒𝑛𝑔𝑡ℎ =𝑚𝑒𝑎𝑠𝑢𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑎𝑟𝑐

360°× 2 𝜋 𝑟

Arc length:

Important corollaries

Corollary (1)

In the same circle (or in congruent circles),

if the measures of arcs are equal, then the lengths of the

arcs are equal, and conversely.

Corollary (2)

In the same circle (or in congruent circles),

if the measures of arcs are equal, then

their chords are equal in length, and

conversely.

Corollary (3)

If two parallel chords are drawn in a circle, then the measure of

the two arcs between them are

equal.

Corollary (4)

If a chord is parallel to a tangent of a circle, then the

measures of the two arcs between them

are equal.

M C

B D A

* *

M

C

B

D

A

M C

B D A

M C

B D

A * *

M

C

B D

A

M

C B

D

A

3rd prep Example (1)

Example (2)

Example (3)

Assignment: Booklet pages 69 & 70