focus on example 3 using angle bisectors careers · 2013-10-20 · 5.1 perpendiculars and bisectors...

Using Angle Bisectors

ROOF TRUSSES Some roofs are builtwith wooden trusses that are assembledin a factory and shipped to the buildingsite. In the diagram of the roof trussshown below, you are given that AB

Æ

bisects ™CAD and that ™ACB and™ADB are right angles. What can yousay about BC

Æand BD

Æ?

SOLUTION

Because BCÆ

and BDÆ

meet ACÆ

and ADÆ

at right angles, they are perpendicularsegments to the sides of ™CAD. This implies that their lengths represent thedistances from the point B to AC

Æand AD

Æ. Because point B is on the bisector of

™CAD, it is equidistant from the sides of the angle.

So, BC = BD, and you can conclude that BCÆ

£ BDÆ

.

1. If D is on the ? of ABÆ

, then D is equidistant from A and B.

2. Point G is in the interior of ™HJK and is equidistant from the sides of the angle, JH

Æand JK

Æ. What can you conclude about G? Use a sketch to support

your answer.

In the diagram, CD¯

is the perpendicular bisector of ABÆ

.

3. What is the relationship between ADÆ

and BDÆ

?

4. What is the relationship between ™ADC and™BDC?

5. What is the relationship between ACÆ

and BCÆ

?Explain your answer.

In the diagram, PMÆ

is the bisector of ™LPN.

6. What is the relationship between ™LPM and ™NPM?

7. How is the distance between point M and PLÆ

related to the distance between point M and PNÆ

?

GUIDED PRACTICE

E X A M P L E 3

5.1 Perpendiculars and Bisectors 267

B

C DA

Vocabulary Check Concept Check

Skill Check

A B

C

D

L

P

N

M

ENGINEERINGTECHNICIAN

In manufacturing,engineering techniciansprepare specifications forproducts such as rooftrusses, and devise and runtests for quality control.

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268 Chapter 5 Properties of Triangles

LOGICAL REASONING Tell whether the information in the diagramallows you to conclude that C is on the perpendicular bisector of AB

Æ.

Explain your reasoning.

8. 9. 10.

LOGICAL REASONING In Exercises 11–13, tell whether the informationin the diagram allows you to conclude that P is on the bisector of ™A.Explain.

11. 12. 13.

14. CONSTRUCTION Draw ABÆ

with a length of 8 centimeters. Construct aperpendicular bisector and draw a point D on the bisector so that the distancebetween D and AB

Æis 3 centimeters. Measure AD

Æand BD

Æ.

15. CONSTRUCTION Draw a large ™A with a measure of 60°. Construct theangle bisector and draw a point D on the bisector so that AD = 3 inches.Draw perpendicular segments from D to the sides of ™A. Measure thesesegments to find the distance between D and the sides of ™A.

USING PERPENDICULAR BISECTORS Use the diagram shown.

16. In the diagram, SV¯

fi RTÆ

and VRÆ

£ VTÆ

. Find VT.


fi RTÆ

and VRÆ

£ VTÆ

. Find SR.


is the perpendicular bisector of RT

Æ. Because UR = UT = 14, what can you

conclude about point U?

USING ANGLE BISECTORS Use the diagram shown.

19. In the diagram, JNÆ

bisects ™HJK, NPÆ

fi JPÆ

,NQÆ

fi JQÆ

, and NP = 2. Find NQ.

20. In the diagram, JNÆ

bisects ™HJK, MHÆ

fi JHÆ

,MKÆ

fi JKÆ

, and MH = MK = 6. What can you conclude about point M?

AP

8

8AP

7

7A P4

3

A

C

P B

A

C

P B

A

C

P B

8 7

PRACTICE AND APPLICATIONS

STUDENT HELP

HOMEWORK HELPExample 1: Exs. 8–10, 14,

16–18, 21–26Example 2: Exs. 11–13,

15, 19, 20, 21–26Example 3: Exs. 31,

33–35

Extra Practiceto help you masterskills is on p. 811.

STUDENT HELP

R

S

U

V

T

8

14

14

17

J

q

P

N

H

M

K

26

6


USING BISECTOR THEOREMS In Exercises 21–26, match the angle measure or segment length described with its correct value.

A. 60° B. 8

C. 40° D. 4

E. 50° F. 3.36

21. SW 22. m™XTV

23. m™VWX 24. VU

25. WX 26. m™WVX

27. PROVING A CONSTRUCTION Write a proof to verify that CP¯

fi ABÆ

inthe construction on page 264.

28. PROVING THEOREM 5.1 Write a proof of Theorem 5.1, thePerpendicular Bisector Theorem. You may want to use the plan for proofgiven on page 265.

GIVEN CP¯

is the perpendicular bisector of ABÆ

.

PROVE C is equidistant from A and B.

29. PROVING THEOREM 5.2 Use the diagram shown to write a two-columnproof of Theorem 5.2, the Converse of the Perpendicular Bisector Theorem.

GIVEN C is equidistant from A and B.

PROVE C is on the perpendicularbisector of AB

Æ.

Plan for Proof Use the Perpendicular Postulate to draw CP

¯fi AB

¯. Show that

¤APC £ ¤BPC by the HL CongruenceTheorem. Then AP

Æ£ BP

Æ, so AP = BP.

30. PROOF Use the diagram shown.

GIVEN GJÆ

is the perpendicular bisector of HK

Æ.

PROVE ¤GHM £ ¤GKM

31. EARLY AIRCRAFT On many of the earliest airplanes, wires connected vertical posts to the edges of the wings, which were wooden frames covered with cloth. Suppose the lengths of the wires from the top of a post to the edges of the frame are the same and the distances from the bottom of the post to the ends of the two wires are the same. What does that tell you about the post and the section of frame between the ends of the wires?

S TV

U

W

X

4

3.3650

30

A B

C

P

Look Back For help with proving thatconstructions are valid,see p. 231.

STUDENT HELP

G J

H

M

K

THE WRIGHTBROTHERS

In Kitty Hawk, NorthCarolina, on December 17,1903, Orville and WilburWright became the firstpeople to successfully fly anengine-driven, heavier-than-air machine.

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32. DEVELOPING PROOF Use the diagram to complete the proof ofTheorem 5.4, the Converse of the Angle Bisector Theorem.

GIVEN D is in the interior of ™ABC andis equidistant from BA

Æand BC

Æ .

PROVE D lies on the angle bisector of ™ABC.

ICE HOCKEY In Exercises 33–35, use the following information.In the diagram, the goalie is at point G and the puck is at point P.The goalie’s job is to prevent the puck from entering the goal.

33. When the puck is at the other end of the rink, the goalie is likely to be standing on line l. How is l related to AB

Æ?

34. As an opposing player with the puck skates toward the goal, the goalie is likely to move from line l to other places on the ice. What should be the relationship between PGÆ

and ™APB?35. How does m™APB change as the puck

gets closer to the goal? Does this change make it easier or more difficult for the goalie to defend the goal? Explain.

36. TECHNOLOGY Use geometry software to construct AB

Æ. Find the midpoint C.

Draw the perpendicular bisector of ABÆ

through C. Construct a point D along theperpendicular bisector and measure DA

Æand DB

Æ.

Move D along the perpendicular bisector. What theorem does this constructiondemonstrate?

1. D is in the interior of ™ABC. 1. ?

2. D is ? from BAÆ

and BCÆ

. 2. Given

3. ? = ? 3. Definition of equidistant

4. DAÆ

fi ?, ? fi BCÆ

4. Definition of distance from a point to a line

5. ? 5. If 2 lines are fi, then they form 4 rt. √.

6. ? 6. Definition of right triangle

7. BDÆ

£ BDÆ

7. ?

8. ? 8. HL Congruence Thm.

9. ™ABD £ ™CBD 9. ?

10. BDÆ

bisects ™ABC and point D 10. ? is on the bisector of ™ABC.

Statements Reasons

BD

A

C

l P

G

A B goalline

goal

5.45.4

AC

B

D


37. MULTI-STEP PROBLEM Use the map shown and the following information.A town planner is trying to decide whether a new household X should be covered byfire station A, B, or C.

a. Trace the map and draw the segments ABÆ

, BCÆ

, and CAÆ

.

b. Construct the perpendicular bisectors of ABÆ

, BCÆ

, and CAÆ

. Do the perpendicular bisectors meet at a point?

c. The perpendicular bisectors divide the town into regions. Shade the region closest to fire station A red. Shade the region closest to fire station B blue. Shade the region closest to fire station C gray.

d. Writing In an emergency at household X, which fire station shouldrespond? Explain your choice.

USING ALGEBRA Use the graph at the right.

38. Use slopes to show thatWSÆ

fi YXÆ

and that WTÆ

fi YZÆ .

39. Find WS and WT.

40. Explain how you know that YWÆ˘

bisects ™XYZ.

CIRCLES Find the missing measurement for the circle shown. Use 3.14 as an approximationfor π. (Review 1.7 for 5.2)

41. radius 42. circumference 43. area

CALCULATING SLOPE Find the slope of the line that passes through thegiven points. (Review 3.6)

44. A(º1, 5), B(º2, 10) 45. C(4, º3), D(º6, 5) 46. E(4, 5), F(9, 5)

47. G(0, 8), H(º7, 0) 48. J(3, 11), K(º10, 12) 49. L(º3, º8), M(8, º8)

USING ALGEBRA Find the value of x. (Review 4.1)

50. 51. 52. 4x

70

(10x 22)

x

40(2x 6)x

31

xyxy

MIXED REVIEW

xyxy

TestPreparation

Challenge y

x

1

1

X (4, 8)

S (3, 5)

Y(2, 2)

T (5, 1)

W (6, 4)

Z (8, 0)

12 cm

EXTRA CHALLENGE

www.mcdougallittell.com

A

B

C

X

5.2 Bisectors of a Triangle 275

1. If three or more lines intersect at the same point, the lines are ?.

2. Think of something about the words incenter and circumcenter that you canuse to remember which special parts of a triangle meet at each point.

Use the diagram and the given information to find the indicated measure.

3. The perpendicular bisectors of 4. The angle bisectors of ¤XYZ meet ¤ABC meet at point G. Find GC. at point M. Find MK.

CONSTRUCTION Draw a large example of the given type of triangle.Construct perpendicular bisectors of the sides. (See page 264.) For the typeof triangle, do the bisectors intersect inside, on, or outside the triangle?

5. obtuse triangle 6. acute triangle 7. right triangle

DRAWING CONCLUSIONS Draw a large ¤ABC.

8. Construct the angle bisectors of ¤ABC. Label the point where the anglebisectors meet as D.

9. Construct perpendicular segments from D to each of the sides of the triangle.Measure each segment. What do you notice? Which theorem have you justconfirmed?

LOGICAL REASONING Use the results of Exercises 5–9 to complete thestatement using always, sometimes, or never.

10. A perpendicular bisector of a triangle ? passes through the midpoint of aside of the triangle.

11. The angle bisectors of a triangle ? intersect at a single point.

12. The angle bisectors of a triangle ? meet at a point outside the triangle.

13. The circumcenter of a triangle ? lies outside the triangle.


X L Z

5

8

Y

K

M

J

12

A

D

B

5

7

C

F

E

G

2

GUIDED PRACTICE


STUDENT HELP

STUDENT HELP

HOMEWORK HELPExample 1: Exs. 5–7,

10–13, 14, 17, 20, 21Example 2: Exs. 8, 9,

10–13, 15, 16, 22

Vocabulary Check

Skill Check

Concept Check


BISECTORS In each case, find the indicated measure.

14. The perpendicular bisectors of 15. The angle bisectors of ¤XYZ meet ¤RST meet at point D. Find DR. at point W. Find WB.

16. The angle bisectors of ¤GHJ 17. The perpendicular bisectors of ¤MNPmeet at point K. Find KB. meet at point Q. Find QN.

ERROR ANALYSIS Explain why the student’s conclusion is false. Then statea correct conclusion that can be deduced from the diagram.

18. 19.

LOGICAL REASONING In Exercises 20 and 21, use the followinginformation and map.Your family is considering moving to a newhome. The diagram shows the locations ofwhere your parents work and where you go toschool. The locations form a triangle.

20. In the diagram, how could you find apoint that is equidistant from eachlocation? Explain your answer.

21. Make a sketch of the situation. Find thebest location for the new home.

J

N

K

L

MP

Q

MQ = MN

A

B

C

D

EF

G

DE = DG

N

P

M

7

48

q

G

H

J

A

B

C

4 5K

X

Y

ZA

B

C

2075

W

S

R T

D

9

16

4.68

school

factory

office

5.2 Bisectors of a Triangle 277

22. DEVELOPING PROOF Complete the proof of Theorem 5.6, theConcurrency of Angle Bisectors.

GIVEN ¤ABC, the bisectors of ™A, ™B, and™C, DE

Æfi AB

Æ, DFÆ

fi BCÆ

, DGÆ

fi CAÆ

PROVE The angle bisectors intersect at a pointthat is equidistant from AB

Æ, BCÆ

, and CAÆ

.

Plan for Proof Show that D, the point ofintersection of the bisectors of ™A and ™B, alsolies on the bisector of ™C. Then show that D isequidistant from the sides of the triangle.

23. Writing Joannie thinks that the midpointof the hypotenuse of a right triangle isequidistant from the vertices of the triangle.Explain how she could use perpendicularbisectors to verify her conjecture.

In Exercises 24–26, use the following information.A mycelium fungus grows underground in all directions from a central point.Under certain conditions, mushrooms sprout up in a ring at the edge. The radiusof the mushroom ring is an indication of the mycelium’s age.

24. Suppose three mushrooms in a mushroom ring are located as shown. Make a large copy of the diagram and draw ¤ABC. Each unit on yourcoordinate grid should represent 1 foot.

25. Draw perpendicular bisectors on your diagram to find the center of the mushroom ring. Estimate the radius of the ring.

26. Suppose the radius of the mycelium increases at a rate of about 8 inches per year. Estimate its age.

SCIENCE CONNECTION

1. ¤ABC, the bisectors of ™A,™B, and ™C, DE

Æfi AB

Æ,

DFÆ

fi BCÆ

, DGÆ

fi CAÆ

2. ? = DG

3. DE = DF

4. DF = DG

5. D is on the ? of ™C.

6. ?

Statements Reasons

1. Given

2. ADÆ

bisects ™BAC, so D is ?from the sides of ™BAC.

3. ?

4. ?

5. Converse of the Angle BisectorTheorem

6. Givens and Steps ?

A(2, 5)

C(4, 1)

B(6, 3)

y

x

1

1

A B

C

D

E

FG

R

ST

q

MUSHROOMS livefor only a few days.

As the mycelium spreadsoutward, new mushroomrings are formed. Amushroom ring in France is almost half a mile indiameter and is about 700 years old.

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MULTIPLE CHOICE Choose the correct answer from the list given.

27. ADÆ

and CDÆ

are angle bisectors of ¤ABCand m™ABC = 100°. Find m™ADC.

¡A 80° ¡B 90° ¡C 100°

¡D 120° ¡E 140°

28. The perpendicular bisectors of ¤XYZintersect at point W, WT = 12, and WZ = 13. Find XY.

¡A 5 ¡B 8 ¡C 10

¡D 12 ¡E 13

USING ALGEBRA Use the graph of ¤ABC to illustrate Theorem 5.5, theConcurrency of Perpendicular Bisectors.

29. Find the midpoint of each side of ¤ABC.Use the midpoints to find the equationsof the perpendicular bisectors of ¤ABC.

30. Using your equations from Exercise 29,find the intersection of two of the lines.Show that the point is on the third line.

31. Show that the point in Exercise 30 is equidistant from the vertices of ¤ABC.

FINDING AREAS Find the area of the triangle described. (Review 1.7 for 5.3)

32. base = 9, height = 5 33. base = 22, height = 7

WRITING EQUATIONS The line with the given equation is perpendicular toline j at point P. Write an equation of line j. (Review 3.7)

34. y = 3x º 2, P(1, 4) 35. y = º2x + 5, P(7, 6)

36. y = º23x º 1, P(2, 8) 37. y =

1101x + 3, P(º2, º9)

LOGICAL REASONING Decide whether enough information is given toprove that the triangles are congruent. If there is enough information, tellwhich congruence postulate or theorem you would use. (Review 4.3, 4.4, and 4.6)

38. 39. 40. K

L

8 MP

N

10

8

10

F

GJ

H

5

5

A B

DE

C

MIXED REVIEW

xyxy

TestPreparation

Challenge

EXTRA CHALLENGE


y

8

2

A (0, 0) C (18, 0)

B (12, 6)

x

100

A

B

C

D

X

Y Z

W

T

12

13


1. The centroid of a triangle is the point where the three ? intersect.

2. In Example 3 on page 281, explain why the two legs of the right triangle inpart (b) are also altitudes of the triangle.

Use the diagram shown and the given information to decide in each casewhether EG

Æis a perpendicular bisector, an angle bisector, a median, or an

altitude of ¤DEF.

3. DGÆ

£ FGÆ

4. EGÆ

fi DFÆ

5. ™DEG £ ™FEG

6. EGÆ

fi DFÆ

and DGÆ

£ FGÆ

7. ¤DGE £ ¤FGE

USING MEDIANS OF A TRIANGLE In Exercises 8–12, use the figure below andthe given information.P is the centroid of ¤DEF, EH

Æfi DF

Æ,

DH = 9, DG = 7.5, EP = 8, and DE = FE.

8. Find the length of FHÆ

.

9. Find the length of EHÆ

.

10. Find the length of PHÆ

.

11. Find the perimeter of ¤DEF.

12. LOGICAL REASONING In the diagram of ¤DEF above, EE

HP =

23.

Find PEHH and PE

HP.

PAPER FOLDING Cut out a large acute, right, or obtuse triangle. Label thevertices. Follow the steps in Exercises 13–16 to verify Theorem 5.7.

13. Fold the sides to locate the midpoint of each side. Label the midpoints.

14. Fold to form the median from each vertex to the midpoint of the opposite side.

15. Did your medians meet at about the same point? If so, label this centroid point.

16. Verify that the distance from the centroid to a vertex is two thirds of the distance from that vertex to the midpoint of the opposite side.


GUIDED PRACTICE

Vocabulary Check Concept Check

Skill Check

STUDENT HELP

HOMEWORK HELPExample 1: Exs. 8–11,

13–16Example 2: Exs. 17–23Example 3: Exs. 24–26


STUDENT HELP

D G F

E

D H F

JG

E

9

7.5P

8

A

BC

L M

N

5.3 Medians and Altitudes of a Triangle 283

USING ALGEBRA Use the graph shown.

17. Find the coordinates of Q, themidpoint of MN

Æ.

18. Find the length of the median PQÆ

.

19. Find the coordinates of thecentroid. Label this point as T.

20. Find the coordinates of R, themidpoint of MP

Æ. Show that the

quotient NN

RT is

23.

USING ALGEBRA Refer back to Example 2 on page 280.

21. Find the coordinates of M, the midpoint of KLÆ

.

22. Use the Distance Formula to find the lengths of JPÆ

and JMÆ

.

23. Verify that JP = 23JM.

CONSTRUCTION Draw and label a large scalene triangle of the giventype and construct the altitudes. Verify Theorem 5.8 by showing that thelines containing the altitudes are concurrent, and label the orthocenter.

24. an acute ¤ABC 25. a right ¤EFG with 26. an obtuse ¤KLMright angle at G

TECHNOLOGY Use geometry software to draw a triangle. Label thevertices as A, B, and C.

27. Construct the altitudes of ¤ABC by drawing perpendicular lines througheach side to the opposite vertex. Label them AD

Æ, BEÆ

, and CFÆ

.

28. Find and label G and H, the intersections of ADÆ

and BEÆ

and of BEÆ

and CFÆ

.

29. Prove that the altitudes are concurrent by showing that GH = 0.

ELECTROCARDIOGRAPH In Exercises 30–33, use the followinginformation about electrocardiographs.The equilateral triangle ¤BCD is used to plot electrocardiograph readings.Consider a person who has a left shoulder reading (S) of º1, a right shoulderreading (R) of 2, and a left leg reading (L) of 3.

30. On a large copy of ¤BCD, plot thereading to form the vertices of ¤SRL.(This triangle is an Einthoven’sTriangle, named for the inventor of theelectrocardiograph.)

31. Construct the circumcenter M of ¤SRL.

32. Construct the centroid P of ¤SRL.Draw line r through P parallel to BC

Æ.

33. Estimate the measure of the acute anglebetween line r and MP

Æ. Cardiologists

call this the angle of a person’s heart.

xyxy

xyxy

2

y

x10

R

œ

P (5, 6)

M (1, 2)

N (11, 2)

B

D

C2 0 2

Right shoulder44

2

0

2

4

4

2

0

2

4

4

Leftshoulder

Leftleg

Look Back To construct an altitude,use the construction of aperpendicular to a linethrough a point not on theline, as shown on p. 130.

STUDENT HELP

CARDIOLOGYTECHNICIAN

Technicians use equipmentlike electrocardiographs totest, monitor, and evaluateheart function.

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34. MULTI-STEP PROBLEM Recall the formula for the area of a triangle,

A = 12bh, where b is the length of the base and h is the height. The height of

a triangle is the length of an altitude.

a. Make a sketch of ¤ABC. Find CD, the height ofthe triangle (the length of the altitude to side AB

Æ).

b. Use CD and AB to find the area of ¤ABC.

c. Draw BEÆ

, the altitude to the line containing side AC

Æ.

d. Use the results of part (b) to find the length of BE

Æ.

e. Writing Write a formula for the length of an altitude in terms of the baseand the area of the triangle. Explain.

SPECIAL TRIANGLES Use the diagram at the right.

35. GIVEN ¤ABC is isosceles. BDÆ

is a median to base ACÆ

.

PROVE BDÆ

is also an altitude.

36. Are the medians to the legs of an isosceles triangle also altitudes? Explain your reasoning.

37. Are the medians of an equilateral triangle also altitudes? Are they containedin the angle bisectors? Are they contained in the perpendicular bisectors?

38. LOGICAL REASONING In a proof, if you are given a median of anequilateral triangle, what else can you conclude about the segment?

USING ALGEBRA Write an equation of the line that passes through point P and is parallel to the line with the given equation. (Review 3.6 for 5.4)

39. P(1, 7), y = ºx + 3 40. P(º3, º8), y = º2x º 3

41. P(4, º9), y = 3x + 5 42. P(4, º2), y = º12x º 1

DEVELOPING PROOF In Exercises 43 and 44, state the third congruencethat must be given to prove that ¤DEF £ ¤GHJ using the indicatedpostulate or theorem. (Review 4.4)

43. GIVEN ™D £ ™G, DFÆ

£ GJÆ

AAS Congruence Theorem

44. GIVEN ™E £ ™H, EFÆ

£ HJÆ

ASA Congruence Postulate

45. USING THE DISTANCE FORMULA Place a right triangle with legsof length 9 units and 13 units in a coordinate plane and use theDistance Formula to find the length of the hypotenuse. (Review 4.7)

xyxy

MIXED REVIEW

TestPreparation

A

E

D B

C

12 8

15

A

D

C

B

G

H

J

D

E

F

Challenge

EXTRA CHALLENGE


5.3 Medians and Altitudes of a Triangle 285

Use the diagram shown and the given information. (Lesson 5.1)HJÆ

is the perpendicular bisector of KLÆ

.

HJÆ

bisects ™KHL.

1. Find the value of x.

2. Find the value of y.

In the diagram shown, the perpendicular bisectors of ¤RST meet at V. (Lesson 5.2)

3. Find the length of VTÆ

.

4. What is the length of VSÆ

? Explain.

5. BUILDING A MOBILE Suppose you want to attach the items in a mobile so that they hang horizontally. You would want to find the balancing point ofeach item. For the triangular metal plate shown, describe where the balancing point would be located. (Lesson 5.3)

QUIZ 1 Self-Test for Lessons 5.1–5.3

K LJ

H

y 24

3x 25

3y

4x 9

V

R S

T

8

6

AE

CF

B

DG

ADÆ

, BEÆ

, and CFÆ

are medians. CF = 12 in.

Optimization

THROUGHOUT HISTORY, people have faced problems involving minimizing resources or maximizing output, a process called optimization. The use of mathematics in solving these types of problems has increased greatly since World War II, when mathematicians found the optimal shape for naval convoys to avoid enemy fire.

TODAY, with the help of computers, optimization techniques are used in many industries, including manufacturing, economics, and architecture.

1. Your house is located at point H in the diagram. You need to do errands at the post office (P), the market (M), and the library (L). In what order should you do your errands to minimize the distance traveled?

2. Look back at Exercise 34 on page 270. Explain why the goalie’s position on the angle bisector optimizes the chances of blocking a scoring shot.

THENTHEN

NOWNOW

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WWII naval convoy

Johannes Kepler proposesthe optimal way to stackcannonballs.

This Olympic stadiumroof uses a minimumof materials.

Thomas Hales proves Kepler’scannonball conjecture.

1972

1942

1611

P

M

L

H

1997


1. In ¤ABC, if M is the midpoint of ABÆ

, N is the midpoint of ACÆ

, and P is themidpoint of BC

Æ, then MN

Æ, NPÆ

, and PNÆ

are ? of ¤ABC.

2. In Example 3 on page 288, why was it convenient to position one of the sidesof the triangle along the x-axis?

In Exercises 3–9, GHÆ

, HJÆ

, and JGÆ

aremidsegments of ¤DEF.

3. JHÆ

∞ ? 4. ? ∞ DEÆ

5. EF = ? 6. GH = ?

7. DF = ? 8. JH = ?

9. Find the perimeter of ¤GHJ.

WALKWAYS The triangle below shows a section of walkways on acollege campus.

10. The midsegment ABÆ

represents a newwalkway that is to be constructed on thecampus. What are the coordinates of points A and B?

11. Each unit in the coordinate plane represents10 yards. Use the Distance Formula to find the length of the new walkway.

COMPLETE THE STATEMENT In Exercises 12–19, use ¤ABC, where L, M,and N are midpoints of the sides.

12. LMÆ ∞ ?

13. ABÆ ∞ ?

14. If AC = 20, then LN = ? .

15. If MN = 7, then AB = ? .

16. If NC = 9, then LM = ? .

17. USING ALGEBRA If LM = 3x + 7 and BC = 7x + 6, then LM = ? .

18. USING ALGEBRA If MN = x º 1 and AB = 6x º 18, then AB = ? .

19. LOGICAL REASONING Which angles in the diagram are congruent?Explain your reasoning.

20. CONSTRUCTION Use a straightedge to draw a triangle. Then use thestraightedge and a compass to construct the three midsegments of the triangle.

xyxy

xyxy


GUIDED PRACTICE


STUDENT HELP

STUDENT HELP

HOMEWORK HELPExample 1: Exs. 21, 22Example 2: Exs. 12–16Example 3: Exs. 23–25Example 4: Exs. 26, 27Example 5: Exs. 28, 29

Vocabulary Check

Skill Check

Concept Check

24

D

G

F

H

EJ

8 10.6

y

x

2

6O (0, 0)

œ (2, 8)

R (10, 4)AB

B

A

L N

M C

5.4 Midsegment Theorem 291

USING ALGEBRA Use the diagram.

21. Find the coordinates of the endpointsof each midsegment of ¤ABC.

22. Use slope and the Distance Formula toverify that the Midsegment Theorem is true for DF

Æ.

USING ALGEBRA Copy the diagram in Example 3 on page 288 tocomplete the proof of Theorem 5.9, the Midsegment Theorem.

23. Locate the midpoint of ABÆ

and label it F. What are the coordinates of F?Draw midsegments DF

Æand EF

Æ.

24. Use slopes to show that DFÆ

∞ CBÆ

and EFÆ

∞ CAÆ

.

25. Use the Distance Formula to find DF, EF, CB, and CA. Verify that

DF = 12CB and EF =

12CA.

USING ALGEBRA In Exercises 26 and 27, you are given the midpoints ofthe sides of a triangle. Find the coordinates of the vertices of the triangle.

26. L(1, 3), M(5, 9), N(4, 4) 27. L(7, 1), M(9, 6), N(5, 4)

FINDING PERIMETER In Exercises 28 and 29, use the diagram shown.

28. Given CD = 14, GF = 8, 29. Given PQ = 20, SU = 12, and GC = 5, find the perimeter and QU = 9, find the perimeter of ¤BCD. of ¤STU.

30. TECHNOLOGY Use geometry software to draw any ¤ABC. Constructthe midpoints of AB

Æ, BCÆ

, and CAÆ

. Label them as D, E, and F. Constructthe midpoints of DE

Æ, EFÆ

, and FDÆ

. Label them as G, H, and I. What is therelationship between the perimeters of ¤ABC and ¤GHI?

31. FRACTALS The design below, which approximates a fractal, is created withmidsegments. Beginning with any triangle, shade the triangle formed by thethree midsegments. Continue the process for each unshaded triangle.Suppose the perimeter of the original triangle is 1. What is the perimeter ofthe triangle that is shaded in Stage 1? What is the total perimeter of all thetriangles that are shaded in Stage 2? in Stage 3?

P

q

RT

S U

B

C

D

G F

E

xyxy

xyxy

xyxy

Stage 0 Stage 1 Stage 2 Stage 3

10

y

x

6

A (0, 2)

C (10, 6)

B (5, 2)D

E

F

HOMEWORK HELPVisit our Web site

www.mcdougallittell.comfor help with Exs. 26 and 27.

INTE

RNET

STUDENT HELP

FRACTALS areshapes that look the

same at many levels ofmagnification. Take a smallpart of the image above andyou will see that it looksabout the same as the whole image.

APPLICATION LINKwww.mcdougallittell.com

INTE

RNET

RE

AL LIFE

RE

AL LIFEFOCUS ON

APPLICATIONS


32. PORCH SWING You are assemblingthe frame for a porch swing. Thehorizontal crossbars in the kit youpurchased are each 30 inches long. Youattach the crossbars at the midpoints ofthe legs. At each end of the frame, howfar apart will the bottoms of the legs bewhen the frame is assembled? Explain.

33. WRITING A PROOF Write a paragraph proof using the diagram shownand the given information.

GIVEN ¤ABC with midsegments DEÆ

, EFÆ

, and FDÆ

PROVE ¤ADE £ ¤DBF

Plan for Proof Use the SAS Congruence Postulate. Show that AD

Æ£ DB

Æ. Show that

because DE = BF = 12BC, then DE

Æ£ BF

Æ.

Use parallel lines to show that ™ADE £ ™ABC.

34. WRITING A PLAN Using the information from Exercise 33, write a planfor a proof showing how you could use the SSS Congruence Postulate toprove that ¤ADE £ ¤DBF.

35. A-FRAME HOUSE In the A-framehouse shown, the floor of the second level,labeled PQ

Æ, is closer to the first floor, RS

Æ,

than midsegment MNÆ

is. If RSÆ

is 24 feetlong, can PQ

Æbe 10 feet long? 12 feet long?

14 feet long? 24 feet long? Explain.

36. MULTI-STEP PROBLEM The diagram below shows the points D(2, 4), E(3, 2),and F(4, 5), which are midpoints of the sides of ¤ABC. The directions belowshow how to use equations of lines to reconstruct the original ¤ABC.

a. Plot D, E, and F in a coordinate plane.

b. Find the slope m1 of one midsegment, sayDEÆ

.

c. The line containing side CBÆ

will have thesame slope as DE

Æ. Because CB

Æcontains

F(4, 5), an equation of CB¯

in point-slopeform is y º 5 = m1(x º 4). Write an equation of CB

¯.

d. Find the slopes m2 and m3 of the other twomidsegments. Use these slopes to findequations of the lines containing the other two sides of ¤ABC.

e. Rewrite your equations from parts (c) and (d) in slope-intercept form.

f. Use substitution to solve systems of equations to find the intersection ofeach pair of lines. Plot these points A, B, and C on your graph.

TestPreparation

B

A

CF

ED

y

x

D (2, 4)F (4, 5)

E (3, 2)A

B

C

crossbar

?

Skills ReviewFor help with writingan equation of a line,see page 795.

STUDENT HELP

5.4 Midsegment Theorem 293

37. FINDING A PATTERN In ¤ABC, the length ofABÆ

is 24. In the triangle, a succession ofmidsegments are formed.

• At Stage 1, draw the midsegment of ¤ABC.Label it DE

Æ.

• At Stage 2, draw the midsegment of ¤DEC.Label it FG

Æ.

• At Stage 3, draw the midsegment of ¤FGC.Label it HJ

Æ.

Copy and complete the table showing the length of the midsegment at each stage.

38. USING ALGEBRA In Exercise 37, let y represent the length of themidsegment at Stage n. Construct a scatter plot for the data given in the table.Then find a function that gives the length of the midsegment at Stage n.

SOLVING EQUATIONS Solve the equation and state a reason for each step. (Review 2.4)

39. x º 3 = 11 40. 3x + 13 = 46

41. 8x º 1 = 2x + 17 42. 5x + 12 = 9x º 4

43. 2(4x º 1) = 14 44. 9(3x + 10) = 27

45. º2(x + 1) + 3 = 23 46. 3x + 2(x + 5) = 40

USING ALGEBRA Find the value of x. (Review 4.1 for 5.5)

47. 48. 49.

ANGLE BISECTORS ADÆ

, BDÆ

, and CDÆ

are angle bisectors of ¤ABC.(Review 5.2)

50. Explain why ™CAD £ ™BAD and™BCD £ ™ACD.

51. Is point D the circumcenter or incenterof ¤ABC?

52. Explain why DEÆ

£ DGÆ

£ DFÆ

.

53. Suppose CD = 10 and EC = 8. Find DF.

(7x 7)61

4x

(7x 1) 38

(10x 22)(x 2)

132

x

xyxy

MIXED REVIEW

xyxy

A B

D E

F G

H J

24

C

A

B

G

F

D

E

C

Stage n 0 1 2 3 4 5

Midsegment length 24 ? ? ? ? ?

Challenge

EXTRA CHALLENGE


5.6 Indirect Proof and Inequalities in Two Triangles 305

1. Explain why an indirect proof might also be called a proof by contradiction.

2. To use an indirect proof to show that two lines m and n are parallel, you would first make the assumption that ?.

In Exercises 3–5, complete with <, >, or =.

3. m™1 ? m™2 4. KL ? NQ 5. DC ? FE

6. Suppose that in a ¤ABC, you want to prove that BC > AC. What are the two cases you would use in an indirect proof?

USING THE HINGE THEOREM AND ITS CONVERSE Complete with <, >, or =.

7. RS ? TU 8. m™1 ? m™2 9. m™1 ? m™2

10. XY ? ZY 11. m™1 ? m™2 12. m™1 ? m™2

13. AB ? CB 14. UT ? SV 15. m™1 ? m™2

9 8

1

2

S T

UV

44

45

20

18B

A

E

D

C

1311

1 21

2W Y

Z

X

3841

1

2

1513

1

2

110

R

S

130

U

T


45L M

K P47

q

N

1 227 26

GUIDED PRACTICE

Concept Check

Skill Check

Vocabulary Check

STUDENT HELP

HOMEWORK HELPExample 1: Exs. 21–24 Example 2: Exs. 25–27 Example 3: Exs. 7–17 Example 4: Exs. 28, 29


STUDENT HELP

37

D

C

E

F

38


LOGICAL REASONING In Exercises 16 and 17, match the giveninformation with conclusion A, B, or C. Explain your reasoning.

A. AD > CD B. AC > BD C. m™4 < m™5

16. AC > AB, BD = CD 17. AB = DC, m™3 < m™5

USING ALGEBRA Use an inequality to describe a restriction on thevalue of x as determined by the Hinge Theorem or its converse.

18. 19. 20.

ASSUMING THE NEGATION OF THE CONCLUSION In Exercises 21–23, writethe first statement for an indirect proof of the situation.

21. If RS + ST ≠ 12 in. and ST = 5 in., then RS ≠ 7 in.

22. In ¤MNP, if Q is the midpoint of NPÆ

, then MQÆ

is a median.

23. In ¤ABC, if m™A + m™B = 90°, then m™C = 90°.

24. DEVELOPING PROOF Arrange statements A–D in correct order to writean indirect proof of Postulate 7 from page 73: If two lines intersect, then theirintersection is exactly one point.

GIVEN line m, line n

PROVE Lines m and n intersect in exactly one point.

A. But this contradicts Postulate 5, which states that there is exactly one linethrough any two points.

B. Then there are two lines (m and n) through points P and Q.

C. Assume that there are two points, P and Q, where m and n intersect.

D. It is false that m and n can intersect in two points, so they must intersectin exactly one point.

25. PROOF Write an indirect proof of Theorem 5.11 on page 295.

GIVEN m™D > m™E

PROVE EF > DF

Plan for Proof In Case 1, assume that EF < DF.In Case 2, assume that EF = DF. Show that neither case can be true, so EF > DF.

65

(4x 5)

3

32

412

12

3x 1

x 3

45

115

9

8

8

x

7060

xyxy

1 4

3 2

5

6

A

B D

C

1 4

3 2

5

6

A

B D

C

HOMEWORK HELPVisit our Web site

www.mcdougallittell.comfor help with negations inExs. 21–23.

INTE

RNET

STUDENT HELP

D E

F

5.6 Indirect Proof and Inequalities in Two Triangles 307

PROOF Write an indirect proof in paragraph form. The diagrams, whichillustrate negations of the conclusions, may help you.

COMPARING DISTANCES In Exercises 28 and 29, consider the flightpaths described. Explain how to use the Hinge Theorem to determine whois farther from the airport.

28. Your flight: 100 miles due west, then 50 miles N 20° WFriend’s flight: 100 miles due north, then 50 miles N 30° E

29. Your flight: 210 miles due south, then 80 miles S 70° WFriend’s flight: 80 miles due north, then 210 miles N 50° E

30. MULTI-STEP PROBLEM Use the diagram of the tank cleaning system’sexpandable arm shown below.

a. As the cleaning system arm expands, EDÆ

gets longer. As ED increases,what happens to m™EBD? What happens to m™DBA?

b. Name a distance that decreases as EDÆ

gets longer.

c. Writing Explain how the cleaning arm illustrates the Hinge Theorem.

31. PROOF Prove Theorem 5.14, the Hinge Theorem.

GIVEN ABÆ

£ DEÆ

, BCÆ

£ EFÆ

, m™ABC > m™DEF

PROVE AC > DF

Plan for Proof

1. Locate a point P outside ¤ABC so you can construct ¤PBC £ ¤DEF.

2. Show that ¤PBC £ ¤DEF by the SAS Congruence Postulate.

3. Because m™ABC > m™DEF, locate a point H on ACÆ

so that BHÆ

bisects ™PBA.

4. Give reasons for each equality or inequality below to show that AC > DF.

AC = AH + HC = PH + HC > PC = DF

26. GIVEN ™1 and ™2 aresupplementary.

PROVE m ∞ n

27. GIVEN RUÆ

is an altitude,RUÆ

bisects ™SRT.

PROVE ¤RST is isosceles.

US T

Rn m

1

23

t

TestPreparation

Challenge

EXTRA CHALLENGE


AB

E D

Begin by assuming that m Ω n. Begin by assuming that RS > RT.

B

A H

P

C E F

D


CLASSIFYING TRIANGLES State whether the triangle described isisosceles, equiangular, equilateral, or scalene. (Review 4.1 for 6.1)

32. Side lengths: 33. Side lengths: 34. Side lengths:3 cm, 5 cm, 3 cm 5 cm, 5 cm, 5 cm 5 cm, 6 cm, 8 cm

35. Angle measures: 36. Angle measures: 37. Angle measures:30°, 30°, 120° 60°, 60°, 60° 65°, 50°, 65°

USING ALGEBRA In Exercises 38–41, use the diagramshown at the right. (Review 4.1 for 6.1)

38. Find the value of x. 39. Find m™B.

40. Find m™C. 41. Find m™BAC.

42. DESCRIBING A SEGMENT Draw any equilateral triangle ¤RST. Draw a line segment from vertex R to the midpoint of side ST

Æ. State everything that you know

about the line segment you have drawn. (Review 5.3)

In Exercises 1–3, use the triangle shown at theright. The midpoints of the sides of ¤CDE are F, G,and H. (Lesson 5.4)

1. FGÆ

∞ ?

2. If FG = 8, then CE = ?.

3. If the perimeter of ¤CDE = 42, then the perimeter of ¤GHF = ?.

In Exercises 4–6, list the sides in order from shortest to longest. (Lesson 5.5)

4. 5. 6.

7. In ¤ABC and ¤DEF shown at the right, which is longer, AB

Æor DE

Æ? (Lesson 5.6)

8. HIKING Two groups of hikers leave from the same base camp and headin opposite directions. The first group walks 4.5 miles due east, then changesdirection and walks E 45° N for 3 miles. The second group walks 4.5 milesdue west, then changes direction and walks W 20° S for 3 miles. Each grouphas walked 7.5 miles, but which is farther from the base camp? (Lesson 5.6)

L M

q

75

74

QUIZ 2 Self-Test for Lessons 5.4–5.6

xyxy

MIXED REVIEW

A

B

D

C3x

(x 13)

(x 19)

C F D

G

E

H

DC

FE73

72B

A

75

M

P

N48

Hikers in the Grand Canyon

50

M

Pq49

focus on example 3 using angle bisectors careers · 2013-10-20 · 5.1 perpendiculars and bisectors...

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