10.6 Segment Relationships in Circles - Big Ideas Learning

10.6 Segment Relationships in Circles

Essential Question

What relationships exist among the

segments formed by two intersecting chords or among segments of two

secants that intersect outside a circle?

Segments Formed by Two Intersecting Chords

Work with a partner. Use dynamic geometry software.

¡ª and DE

¡ª

a. Construct two chords BC

that intersect in the interior of a

circle at a point F.

Sample

REASONING

ABSTRACTLY

To be proficient in math,

you need to make sense

of quantities and their

relationships in problem

situations.

E

B

b. Find the segment lengths BF, CF,

DF, and EF and complete the table.

What do you observe?

F

?

BF

CF

BF CF

DF

EF

DF EF

A

C

D

?

c. Repeat parts (a) and (b) several times. Write a conjecture about your results.

Secants Intersecting Outside a Circle

Work with a partner. Use dynamic geometry software.

??

a. Construct two secants ??

BC and BD

that intersect at a point B outside

a circle, as shown.

Sample

C

b. Find the segment lengths BE, BC,

BF, and BD, and complete the table.

What do you observe?

A

?

BE

BC

BE BC

BF

BD

BF BD

E

B

F

D

?

c. Repeat parts (a) and (b) several times. Write a conjecture about your results.

Communicate Your Answer

D

E

9

A

18

3. What relationships exist among the segments formed by two intersecting chords

or among segments of two secants that intersect outside a circle?

F

4. Find the segment length AF in the figure at the left.

8

C

Section 10.6

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10.6 Lesson

What You Will Learn

Use segments of chords, tangents, and secants.

Core Vocabul

Vocabulary

larry

segments of a chord, p. 570

tangent segment, p. 571

secant segment, p. 571

external segment, p. 571

Using Segments of Chords, Tangents, and Secants

When two chords intersect in the interior of a circle, each chord is divided into two

segments that are called segments of the chord.

Theorem

Theorem 10.18 Segments of Chords Theorem

If two chords intersect in the interior of a circle, then

the product of the lengths of the segments of one chord

is equal to the product of the lengths of the segments of

the other chord.

C

A

E

D

B

EA ? EB = EC ? ED

Proof Ex. 19, p. 574

Using Segments of Chords

M

Find ML and JK.

x+2

N

K x

?

?

NK NJ = NL NM

?

x (x + 4) = (x + 1) (x + 2)

x2

J

L

SOLUTION

?

x+4

x+1

+ 4x =

x2

+ 3x + 2

4x = 3x + 2

x=2

Segments of Chords Theorem

Substitute.

Simplify.

Subtract x2 from each side.

Subtract 3x from each side.

Find ML and JK by substitution.

ML = (x + 2) + (x + 1)

JK = x + (x + 4)

=2+2+2+1

=2+2+4

=7

=8

So, ML = 7 and JK = 8.

Monitoring Progress

Help in English and Spanish at

Find the value of x.

1.

2.

x 6

4 3

570

Chapter 10

hs_geo_pe_1006.indd 570

2

4

x+1

3

Circles

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Core Concept

Tangent Segment and Secant Segment

R

external segment Q

secant segment

P

A tangent segment is a segment that

is tangent to a circle at an endpoint.

A secant segment is a segment that

contains a chord of a circle and has

exactly one endpoint outside the

circle. The part of a secant segment

that is outside the circle is called an

external segment.

tangent segment

S

PS is a tangent segment.

PR is a secant segment.

PQ is the external segment of PR.

Theorem

Theorem 10.19 Segments of Secants Theorem

If two secant segments share the same endpoint

outside a circle, then the product of the lengths of

one secant segment and its external segment equals the

product of the lengths of the other secant segment and

its external segment.

A

B

E

C

D

EA ? EB = EC ? ED

Proof Ex. 20, p. 574

Using Segments of Secants

Find the value of x.

R

P

9

10

Q

11

x

S

T

SOLUTION

?

?

9 ? (11 + 9) = 10 ? (x + 10)

RP RQ = RS RT

180 = 10x + 100

80 = 10x

8=x

Segments of Secants Theorem

Substitute.

Simplify.

Subtract 100 from each side.

Divide each side by 10.

The value of x is 8.

Monitoring Progress

Help in English and Spanish at

Find the value of x.

3.

4.

9

x

x+1

5

Section 10.6

hs_geo_pe_1006.indd 571

3

x+2

6

x?1

Segment Relationships in Circles

571

1/19/15 2:38 PM

Theorem

Theorem 10.20 Segments of Secants and Tangents Theorem

A

If a secant segment and a tangent segment share an

endpoint outside a circle, then the product of the lengths

of the secant segment and its external segment equals

E

the square of the length of the tangent segment.

C

D

?

Proof Exs. 21 and 22, p. 574

EA2 = EC ED

Using Segments of Secants and Tangents

Find RS.

SOLUTION

ANOTHER WAY

In Example 3, you can draw

¡ª and QT

¡ª.

segments QS

Q

16

R

x

?

x

Segments of Secants

and Tangents Theorem

?

RQ2 = RS RT

162 = x (x + 8)

Substitute.

Simplify.

256 =

x2

+ 8x

0=

x2

+ 8x ? 256

Q

16

R

8

S

T

Write in standard form.

¡ª¡ª

S

?8 ¡À ¡Ì 82 ? 4(1)(?256)

x = ¡ª¡ª¡ª

Use Quadratic Formula.

2(1)

8

T

Because ¡ÏRQS and ¡ÏRTQ

intercept the same arc,

they are congruent. By

the Reflexive Property

of Congruence (Theorem

2.2), ¡ÏQRS ? ¡ÏTRQ. So,

¡÷RSQ ¡« ¡÷RQT by the

AA Similarity Theorem

(Theorem 8.3). You can use

this fact to write and solve

a proportion to find x.

¡ª

x = ?4 ¡À 4¡Ì 17

Simplify.

Use the positive solution because lengths cannot be negative.

¡ª

So, x = ?4 + 4¡Ì17 ¡Ö 12.49, and RS ¡Ö 12.49.

Finding the Radius of a Circle

Find the radius of the aquarium tank.

B

20 ft

SOLUTION

?

CB2 = CE CD

Segments of Secants

and Tangents Theorem

?

202 = 8 (2r + 8)

Substitute.

400 = 16r + 64

Simplify.

336 = 16r

Subtract 64 from each side.

21 = r

r

D

r

E

8 ft

C

Divide each side by 16.

So, the radius of the tank is 21 feet.

Monitoring Progress

Help in English and Spanish at

Find the value of x.

5.

3

1

x

6.

5

7

x

x

7.

10

12

8. WHAT IF? In Example 4, CB = 35 feet and CE = 14 feet. Find the radius of

the tank.

572

Chapter 10

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10.6 Exercises

Dynamic Solutions available at

Vocabulary and Core Concept Check

1. VOCABULARY The part of the secant segment that is outside the circle is called a(n) _____________.

2. WRITING Explain the difference between a tangent segment and a secant segment.

Monitoring Progress and Modeling with Mathematics

In Exercises 3¨C6, find the value of x. (See Example 1.)

3.

15. ERROR ANALYSIS Describe and correct the error in

finding CD.

4.

?

x?3

12

10

10

6

x

18

9

5.

8

x+8

2x

?

?

?

12

x+3

8.

x

4

8

9.

10.

45

5

4

7

x

6

x+4

x?2

83,000 km

27

Tethys

In Exercises 11¨C14, find the value of x. (See Example 3.)

11.

x

x

D

Cassini

203,000 km

Telesto

T

l t

x

12

14.

x+4

3

2

x

Section 10.6

hs_geo_pe_1006.indd 573

C

A

24

9

Calypso

B

Saturn

12.

12

?

?

x

50

7

C

spacecraft is on a mission in orbit around Saturn until

September 2017. Three of Saturn¡¯s moons, Tethys,

Calypso, and Telesto, have nearly circular orbits of

radius 295,000 kilometers. The diagram shows the

positions of the moons and the spacecraft on one

of Cassini¡¯s missions. Find the distance DB from

¡ª is tangent to the circular

Cassini to Tethys when AD

orbit. (See Example 4.)

5

10

B

16. MODELING WITH MATHEMATICS The Cassini

In Exercises 7¨C10, find the value of x. (See Example 2.)

7.

D

5

CD DF = AB AF

CD 4 = 5 3

CD 4 = 15

CD = 3.75

15

6

3 A

4

6.

x

13.

F

Segment Relationships in Circles

573

1/19/15 2:38 PM

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