LESSON Reteach Triangles - Scarsdale Public Schools

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LESSON

Class

Reteach

7-5 Triangles

You can classify triangles by the measures of their angles.

An acute triangle has only acute angles.

A right triangle has one right angle.

An obtuse triangle has one obtuse angle.

To classify the triangle below by its angles, you first need to find the

measure of all the angles.

C

46¡ã

The sum of the angles of a triangle is 180¡ã.

34¡ã

A

B

To find the unknown angle, first find the sum of the two known angles.

34 ! 46 " 80

Then subtract the sum from 180.

180 # 80 " 100

The difference is the measure of the third angle.

The measures of the angles are 34¡ã, 46¡ã, and 100¡ã. So the triangle

is obtuse.

Find each unknown angle. Then classify each triangle.

1.

2. A

Y

40¡ã

75¡ã

77¡ã

X

50¡ã

Z

B

3.

4.

90¡ã

30¡ã

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C

60¡ã

60¡ã

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Holt Middle School Math Course 1

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LESSON

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Reteach

7-5 Triangles (continued)

You can also classify triangles by the lengths of their sides.

A scalene triangle has no congruent sides.

An isosceles triangle has at least 2 congruent sides.

An equilateral triangle has 3 congruent sides.

The sum of the lengths of the sides of the triangle below is 18.6 cm.

To classify the triangle below by its sides, you first need to find the

length of the unknown side.

A

7.3 cm

3.5 cm

B

C

?

To find the unknown length, first find the sum of the two known

lengths.

7.3 ! 3.5 " 10.8

Then subtract this sum from the sum of all three sides.

18.6 # 10.8 " 7.8

The length of the third side is 7.8 cm.

The lengths of the sides of the triangle are 3.5 cm, 7.3 cm, and

7.8 cm, so the triangle is scalene.

Find each missing length. Then classify each triangle.

3.

4.

L

6.7 in.

X

6.7 in.

9 in.

J

?

?

K

Y

4 in. Z

The sum of the lengths of the

sides is 22 inches.

The sum of the lengths of the

sides is 20.1 inches.

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Holt Middle School Math Course 1

Practice B

7-5 Triangles

Practice A

7-5 Triangles

LESSON

LESSON

Use the diagram to find the measure of each indicated angle.

Classify each triangle as acute, obtuse, or right.

1.

2.

3.

60¡ã

70¡ã

90¡ã

30¡ã

30¡ã

60¡ã

right

50¡ã

acute

5.

2 in.

2 in.

3 in.

6.

5 cm

5 cm

90¡ã

4. !EBA

90¡ã

5. !ACB

45¡ã

B

10 in.

7. The sum of the lengths

of the three sides is

15 cm.

10 in.

?

70¡ã

45¡ã

8 ft

6 ft

3 cm

equilateral

35¡ã

?

45¡ã

8. The sum of the lengths

of the three sides is

22 ft.

5 cm

9.

50¡ã

90¡ã

C

A

6. The sum of the lengths

of the three sides is

30 in.

scalene

8.

?

45¡ã

45¡ã

Find the measure of the unknown angle.

7.

D

E

Classify each triangle using the given information.

5 ft

equilateral

45¡ã

3. !ABC

3 ft

isosceles

2. !DAC

120¡ã 30¡ã

4 ft

5 cm

90¡ã

obtuse

Classify each triangle as scalene, isosceles, or equilateral.

4.

1. !CBD

60¡ã

scalene

isosceles

9. The angles of a triangular sail measure 90¡ã, 30¡ã, and 60¡ã. Its

sides measure approximately 2 feet, 3.5 feet, and 4 feet. Classify

the triangular shape of the sail in two different ways.

35¡ã

110¡ã

It is a right triangle; It is a scalene triangle.

10. One side of an equilateral triangle measures 4 cm. What are the

lengths of the other two sides of the triangle?

10. Two angles in one triangle are congruent to two angles in

another triangle. What can you conclude about the third angle

in both triangles?

They both measure 4 cm.

They are congruent.

11. Karen says the angles of her triangle measure 90¡ã, 50¡ã, and

60¡ã. Explain why this is impossible.

The sum of those angles is 200¡ã, and the sum of the angles in any

triangle is always 180¡ã.

47

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Holt Middle School Math Course 1

LESSON

Use the diagram to find the measure of each indicated angle.

1. !DEC

79¡ã

B

2. !ACB

52¡ã

70¡ã

3. !BCE

117¡ã

52¡ã

You can classify triangles by the measures of their angles.

An acute triangle has only acute angles.

A right triangle has one right angle.

An obtuse triangle has one obtuse angle.

E

11¡ã

58¡ã

To classify the triangle below by its angles, you first need to find the

measure of all the angles.

11¡ã

A

C

C

D

46¡ã

The lengths of sides !

AB

! and B

!C

! are given for "ABC. Use the

sum of the lengths of the three sides to calculate the length of

side !

AC

! and classify each triangle.

5. AB ! 1.5; BC ! 4.8;

sum ! 10

1

1

6. AB ! 2!4!; BC ! 2!4!;

3

sum ! 6!4!

7. AB ! 638; BC ! 181;

AC ! 181;

scalene

equilateral

isosceles

1

!B ! !2! ? !A

To find the unknown angle, first find the sum of the two known angles.

Then subtract the sum from 180.

180 # 80 ! 100

The difference is the measure of the third angle.

The measures of the angles are 34¡ã, 46¡ã, and 100¡ã. So the triangle

is obtuse.

The measures of !A and !B are given for "ABC. Calculate the

measure of !C and classify each triangle.

9. !A ! 60¡ã;

B

34 " 46 ! 80

sum ! 1,000

AC ! 3.7;

!B ! 136.5¡ã

The sum of the angles of a triangle is 180¡ã.

34¡ã

A

1

AC ! 2!4!;

8. !A ! 22.17¡ã;

Holt Middle School Math Course 1

Reteach

7-5 Triangles

Practice C

7-5 Triangles

LESSON

4. !EBC

48

Copyright ? by Holt, Rinehart and Winston.

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10. !A ! 26.5¡ã;

Find each unknown angle. Then classify each triangle.

!B ! 3 ? !A

!C ! 21.33¡ã;

!C ! 90¡ã;

!C ! 74¡ã;

obtuse

right

acute

1.

2. A

Y

40¡ã

75¡ã

77¡ã

X

11. Starting from the airport, a pilot flew directly north for 112 miles.

Then she made a 90¡ã turn and flew east for 84 miles to reach

Bear Lake. If she flew in a straight line, in which direction did the

pilot fly to get back to the airport? If she flew a total of 336 miles,

how far is the airport from Bear Lake?

C

50¡ã

Z

B

28¡ã, acute

90¡ã, right

3.

4.

60¡ã

She flew southwest to the airport; 140 miles

90¡ã

12. !A of "ABC is congruent to !G of "FGH. !B of "ABC and

!F of "FGH are supplementary. !C measures 29¡ã, and !A

measures 37¡ã. What is the measure of each angle in "FGH?

49

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60¡ã

60¡ã, right

!G ! 37¡ã; !F ! 66¡ã; !H ! 77¡ã

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30¡ã

Holt Middle School Math Course 1

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150

60¡ã, acute

50

Holt Middle School Math Course 1

Holt Middle School Math Course 1

Challenge

7-5 Square Legs

Reteach

7-5 Triangles (continued)

LESSON

LESSON

You can also classify triangles by the lengths of their sides.

About 2,400 years ago a Greek philosopher

and mathematician named Pythagoras proved

a very important rule for triangles. Today

people all over the world study and use this

rule named in his honor¡ªthe Pythagorean

Theorem. It states the relationship between

the side lengths of a right triangle.

A scalene triangle has no congruent sides.

An isosceles triangle has at least 2 congruent sides.

An equilateral triangle has 3 congruent sides.

The sum of the lengths of the sides of the triangle below is 18.6 cm.

To classify the triangle below by its sides, you first need to find the

length of the unknown side.

A

7.3 cm

3.5 cm

B

Pythagoras

C

?

PYTHAGOREAN THEOREM

To find the unknown length, first find the sum of the two known

lengths.

In a right triangle, the square of the length

of the hypotenuse is equal to the sum of

the squares of the lengths of the legs.

7.3 " 3.5 ! 10.8

The legs are the

sides that form the

right angle

Then subtract this sum from the sum of all three sides.

18.6 # 10.8 ! 7.8

2

2

(leg) " (leg) ! (hypotenuse)

The hypoteneuse is

the side opposite

the right angle

2

The length of the third side is 7.8 cm.

a2 " b2 !

The lengths of the sides of the triangle are 3.5 cm, 7.3 cm, and

7.8 cm, so the triangle is scalene.

2

2

3

"

4

9

" 16

c2

c=5

a=3

2

!

5

!

25

b=4

Find each missing length. Then classify each triangle.

3.

4.

L

6.7 in.

X

9 in.

J

Use the Pythagorean Theorem to find the length of the missing

side for each right triangle below.

6.7 in.

?

?

K

4 in. Z

The sum of the lengths of the

sides is 22 inches.

The sum of the lengths of the

sides is 20.1 inches.

6.7 inches, equilateral

c = 13

a=?

Y

c=?

a=7

b = 24

b = 12

9 inches, isosceles

a!5

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51

Holt Middle School Math Course 1

Problem Solving

7-5 Triangles

c ! 25

52

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Holt Middle School Math Course 1

Puzzles, Twisters & Teasers

7-5 Find the Measure

LESSON

LESSON

Use the triangle diagram to answer each question.

1. Classify triangle ABC. What is the

measure of the missing angle?

M

3 cm

"ABC is an acute triangle, 70¡ã

2. Classify triangle XYZ. What is the

measure of the missing angle?

2 in.

3 cm

O

G

N

"XYZ is an obtuse triangle, 100¡ã

Find the missing angle measure in each triangle. When you

have completed the problem, look at the blank lines at the

bottom of the page. Notice if any of the angle degrees of the

letters match the degrees under the blank lines. If they do, write

the letter of the angle on the line. The resulting message will be

the answer to the riddle.

E

4 in.

3 in.

F

X

1.

2.

x¡ã

29¡ã

3. If triangle MNO is an equilateral

triangle, what is the measure of the

missing side?

C

!N

O

! ! 3 cm

50¡ã

60¡ã

B

51¡ã

Z

70¡ã

Y

K

5. Classify triangle EFG.

4. What is the complement of !XYZ?

"EFG is a scalene triangle

39¡ã

70¡ã

x¡ã

J

x ! 40¡ã

x¡ã

T

V

R

5.

Y

x¡ã

87¡ã

30¡ã

C

measures of the angles in any

35¡ã

x ! 105¡ã

x ! 45¡ã

F

120¡ã

Because the sum of the

40¡ã

x¡ã

4.

7. What is the supplement of !ABC?

6. Explain how you can find the

measure of !CAB.

3.

V

A

x¡ã

33¡ã

W

x ! 60¡ã

M

N

x ! 120¡ã

triangle is 180¡ã, subtract the

measure of !ACB and !ABC

from 180¡ã.

6.

7.

S

Circle the letter of the correct answer.

9. Which of the following is not true of

all right triangles?

F The sum of the measures of the

angles is 180¡ã.

G Two of its angles are

!

complementary angles.

H At least two of its angles are

acute.

J The side with the greatest length

is opposite the right angle.

8. Which of the following statements is

always true?

A A right triangle is a scalene

triangle.

B An equilateral triangle is an

!

isosceles triangle.

C An isosceles triangle is an obtuse

triangle.

D A right triangle is an acute

triangle.

60¡ã

30¡ã

Z

x!

53

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Holt Middle School Math Course 1

x¡ã

U

A

50¡ã

I

35¡ã

115¡ã

z ! 55¡ã

L

x ! 68¡ã

y ! 62¡ã

x ¡ã 112¡ã

L

Why do you like studying about volcanoes?

I

50¡ã

Copyright ? by Holt, Rinehart and Winston.

All rights reserved.

y¡ã

z¡ã

T ¡¯

105¡ã

S

A

L

A

V

A

F

U

N

55¡ã

62¡ã

68¡ã

62¡ã

45¡ã

62¡ã

60¡ã

115¡ã

120¡ã

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All rights reserved.

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Holt Middle School Math Course 1

Holt Middle School Math Course 1

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