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Mathematics LibreTexts

1.0: Angles and Triangles

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Historically, trigonometry began as the study of triangles and their properties. Let’s review some definitions and facts from geometry.

  • We measure angles in degrees.
  • One full rotation is 360, as shown below.
  • Half a full rotation is 180 and is called a straight angle.
  • One quarter of a full rotation is 90 and is called a right angle.

Screen Shot 2022-09-08 at 10.06.14 PM.png

Triangles

If you tear off the corners of any triangle and line them up, as shown below, they will always form a straight angle.

Screen Shot 2022-09-08 at 10.07.03 PM.png

Sum of angles in a triangle.

1. The sum of the angles in a triangle is 180.

Example 1.1

Two of the angles in the triangle at right are 25 and 115. Find the third angle.

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Answer

To find the third angle, we write an equation.

x+25+115=180Simplify the left side.x+140=180Subtract 140 from both sides.x=40

The third angle is 40.

Checkpoint 1.2

Find each of the angles in the triangle at right.

Screen Shot 2022-09-08 at 10.13.28 PM.png

Answer

x=39,2x=78,2x15=63

Some special categories of triangles are particularly useful. Most important of these are the right triangles.

Right triangle.

2. A right triangle has one angle of 90.

Example 1.3

One of the smaller angles of a right triangle is 34. What is the third angle?

Screen Shot 2022-09-08 at 10.18.05 PM.png

Answer

The sum of the two smaller angles in a right triangle is 90. So

x+34=90Subtract 34 from both sidesx=56

The unknown angle must be 56.

Checkpoint 1.4

Two angles of a triangle are 35 and 45. Can it be a right triangle?

Answer

No.

An equilateral triangle has all three sides the same length.

Angles of equilateral triangle.

3. All of the angles of an equilateral triangle are equal.

Example 1.5

All three sides of a triangle are 4 feet long. Find the angles.

Screen Shot 2022-09-08 at 10.21.58 PM.png

Answer

The triangle is equilateral, so all of its angles are equal. Thus

3x=180Divide both sides by 3.x=60

Each of the angles is 60.

Checkpoint 1.6

Find x,y, and z in the triangle at right.

Screen Shot 2022-09-08 at 10.25.51 PM.png

Answer

x=60,y=8,z=8

An isosceles triangle has two sides of equal length. The angle between the equal sides is the vertex angle. The other two angles are the base angles.

Base angles of an isoceles triangle.

4. The base angles of an isosceles triangle are equal.

Example 1.7

Find x and y in the triangle at right.

Screen Shot 2022-09-08 at 10.28.26 PM.png

Answer

The triangle is isosceles, so the base angles are equal. Therefore, y=38. To find the vertex angle, we solve

x+38+38=180x+76=180Subtract 76 from both sides.x=104

The vertex angle is 104.

Checkpoint 1.8

Find x and y in the figure at right.

Screen Shot 2022-09-08 at 10.30.50 PM.png

Answer

x=140,y=9

Angles

In addition to the facts about triangles reviewed above, there are several useful properties of angles.

  • Two angles that add to 180 are called supplementary.
  • Two angles that add to 90 are called complementary.
  • Angles between 0 and 90 are called acute.
  • Angles between 90 and 180 are called obtuse.

Screen Shot 2022-09-08 at 10.34.01 PM.png

Example 1.9

In the figure at right,

Screen Shot 2022-09-08 at 10.34.53 PM.png

  • AOC and BOC are supplementary.
  • DOE and BOE are complementary.
  • AOC is obtuse,
  • and BOC is acute.

In trigonometry we often use lower-case Greek letters to represent unknown angles (or, more specifically, the measure of the angle in degrees). In the next Exercise, we use the Greek letters α (alpha), β (beta), and γ (gamma).

Checkpoint 1.10

In the figure, α,β, and γ denote the measures of the angles in degrees.

Screen Shot 2022-09-08 at 10.39.21 PM.png

a. Find the measure of angle α.
b. Find the measure of angle β.
c. Find the measure of angle γ.
d. What do you notice about the measures of the angles?

Answer

α=130,β=50,γ=130. The non-adjacent angles are equal.

Non-adjacent angles formed by the intersection of two straight lines are called vertical angles. In the previous exercise, the angles labeled α and γ are vertical angles, as are the angles labeled β and 50.

Vertical Angles.

5. Vertical angles are equal.

Example 1.11

Explain why α=β in the triangle at right.

Screen Shot 2022-09-08 at 10.42.28 PM.png

Answer

Because they are the base angles of an isosceles triangle, θ (theta) and ϕ(phi) are equal. Also, α=θ because they are vertical angles, and similarly β=ϕ. Therefore, α=β because they are equal to equal quantities.

Checkpoint 1.12

Find all the unknown angles in the figure at right. (You will find a list of all the Greek letters and their names at the end of this section.)

Screen Shot 2022-09-08 at 10.43.59 PM.png

Answer

α=40,β=140,γ=75,δ=65

A line that intersects two parallel lines forms eight angles, as shown in the figure below. There are four pairs of vertical angles, and four pairs of corresponding angles, or angles in the same position relative to the transversal on each of the parallel lines.

For example, the angles labeled 1 and 5 are corresponding angles, as are the angles labeled 4 and 8. Finally, angles 3 and 6 are called alternate interior angles, and so are angles 4 and 5.

Screen Shot 2022-09-08 at 10.45.55 PM.png

Parallel lines cut by a transversal.

6. If parallel lines are intersected by a transversal, the alternate interior angles are equal. Corresponding angles are also equal.

Example 1.13

The parallelogram ABCD shown at right is formed by the intersection of two sets of parallel lines. Show that the opposite angles of the parallelogram are equal.

Screen Shot 2022-09-08 at 10.47.36 PM.png

Answer

Angles 1 and 2 are equal because they are alternate interior angles, and angles 2 and 3 are equal because they are corresponding angles. Therefore angles 1 and 3 , the opposite angles of the parallelogram, are equal. Similarly, you can show that angles 4,5 , and 6 are equal.

Checkpoint 1.14

Show that the adjacent angles of a parallelogram are supplementary. (You can use angles 1 and 4 in the parallelogram of the previous example.)

Answer

Note that angles 2 and 6 are supplementary because they form a straight angle. Angle 1 equals angle 2 because they are alternate interior angles, and similarly angle 4 equals angle 5. Angle 5 equals angle 6 because they are corresponding angles. Thus, angle 4 equals angle 6, and angle 1 equals angle 2. So angles 4 and 1 are supplementary because 2 and 6 are.

Note 1.15 In the Section 1.1 Summary, you will find a list of vocabulary words and a summary of the facts from geometry that we reviewed in this section. You will also find a set of study questions to test your understanding, and a list of skills to practice in the homework problems.

Table 1.16 Lower Case Letters in the Greek Alphabet

 Greek Alphabet α alpha β beta γ gamma δ delta ϵ epsilon γ gamma η eta θ theta ι iota κ kappa λ lambda μ mu ν nu ξ xi o omicron π pi ρ rho σ sigma τ tau v upsilon ϕ phi χ chi ψ psi ω omega 

Review the following skills you will need for this section.

Algebra Refresher 1.2

Solve the equation.

1. x8=192x
2. 2x9=12x
3. 13x+5=2x28
4. 4+9x=7+x

Solve the system.

5. 5x2y=13

2x+3y=9

6. 4x+3y=9

3x+2y=8

Answer

1. 9

2. 2

3. 3

4. 2

5. x=3,y=1

6. x=6,y=5

Section 1.1 Summary

Vocabulary

• Right angle

• Straight angle

• Right triangle

• Equilateral triangle

• Isosceles triangle

• Vertex angle

• Base angle

• Supplementary

• Complementary

• Acute

• Obtuse

• Vertical angles

• Transversal

• Corresponding angles

• Alternate interior angles

Concepts

Facts from Geometry.

1. The sum of the angles in a triangle is 180.
2. A right triangle has one angle of 90.
3. All of the angles of an equilateral triangle are equal.
4. The base angles of an isosceles triangle are equal.
5. Vertical angles are equal.
6. If parallel lines are intersected by a transversal, the alternate interior angles are equal.

Corresponding angles are also equal.

Study Questions

1. Is it possible to have more than one obtuse angle in a triangle? Why or why not?

2. Draw any quadrilateral (a four-sided polygon) and divide it into two triangles by connecting two opposite vertices by a diagonal. What is the sum of the angles in your quadrilateral?

3. What is the difference between a vertex angle and vertical angles?

4. Can two acute angles be supplementary?

5. Choose any two of the eight angles formed by a pair of parallel lines cut by a transversal. Those two angles are either equal or _______ .

Skills

Practice each skill in the Homework Problems listed.

1. Sketch a triangle with given properties #1–6

2. Find an unknown angle in a triangle #7–12, 17–20

3. Find angles formed by parallel lines and a transversal #13–16, 35–44

4. Find exterior angles of a triangle #21–24

5. Find angles in isosceles, equilateral, and right triangles #25–34

6. State reasons for conclusions #45–48

Homework 1.1

For Problems 1–6, sketch and label a triangle with the given properties.

1. An isosceles triangle with vertex angle 30
2. A scalene triangle with one obtuse angle (Scalene means three unequal sides.)
3. A right triangle with legs4 and 7
4. An isosceles right triangle
5. An isosceles triangle with one obtuse angle
6. A right triangle with one angle 20

For Problems 7–20, find each unknown angle.

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8. Screen Shot 2022-09-08 at 11.07.35 PM.png

9. Screen Shot 2022-09-08 at 11.08.03 PM.png

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11. Screen Shot 2022-09-08 at 11.09.16 PM.png

12. Screen Shot 2022-09-08 at 11.09.45 PM.png

13. Screen Shot 2022-09-08 at 11.10.27 PM.png

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19. Screen Shot 2022-09-08 at 11.13.39 PM.png

20. Screen Shot 2022-09-08 at 11.13.58 PM.png

In Problems 21 and 22, the angle labeled ϕ is called an exterior angle of the triangle, formed by one side and the extension of an adjacent side. Find ϕ.

21. Screen Shot 2022-09-08 at 11.15.23 PM.png

22. Screen Shot 2022-09-08 at 11.15.50 PM.png

23. In parts (a) and (b), find the exterior angle ϕ.

a. Screen Shot 2022-09-08 at 11.16.34 PM.png

b. Screen Shot 2022-09-08 at 11.16.56 PM.png

c. Find an algebraic expression for ϕ.

Screen Shot 2022-09-08 at 11.17.29 PM.png

d Use your answer to part (c) to write a rule for finding an exterior angle of a triangle.

24.

a Find the three exterior angles of the triangle. What is the sum of the exterior angles?

Screen Shot 2022-09-08 at 11.18.19 PM.png

b Write an algebraic expression for each exterior angle in terms of one of the angles of the triangle. What is the sum of the exterior angles?

Screen Shot 2022-09-08 at 11.18.49 PM.png

In Problems 25 and 26, the figures inscribed are regular polygons, which means that all their sides are the same length, and all the angles have the same measure. Find the angles θ and ϕ.

25. Screen Shot 2022-09-08 at 11.19.46 PM.png

26. Screen Shot 2022-09-08 at 11.20.11 PM.png

In problems 27 and 28, ΔABC is equilateral. Find the unknown angles

27. Screen Shot 2022-09-08 at 11.21.16 PM.png

28. Screen Shot 2022-09-08 at 11.21.39 PM.png

29. Screen Shot 2022-09-08 at 11.22.04 PM.png

a \boldsymbol{2\theta + 2\phi = ________}

b \boldsymbol{\theta + \phi = ________}

c ΔABC is ________

30. Find α and β.

Screen Shot 2022-09-08 at 11.23.53 PM.png

31. Screen Shot 2022-09-08 at 11.24.23 PM.png

a Explain why OAB and ABO are equal in measure.

b Explain why OBC and BCO are equal in measure.

c Explain why ABC is a right angle. (Hint: Use Problem 29.)

32. Screen Shot 2022-09-08 at 11.26.14 PM.png

a Compare θ with α+β. (Hint: What do you know about supplementary angles and the sum of angles in a triangle?

b Compare α and β.

c Explain why the inscribed angle BAO is half the size of the central angle BOD.

33. Find α and β.

Screen Shot 2022-09-08 at 11.28.04 PM.png

34. Find α and β.

Screen Shot 2022-09-08 at 11.28.41 PM.png

In Problems 35–44, arrows on a pair of lines indicate that they are parallel. Find x and y.

35. Screen Shot 2022-09-08 at 11.29.38 PM.png

36. Screen Shot 2022-09-08 at 11.29.58 PM.png

37. Screen Shot 2022-09-08 at 11.30.27 PM.png

38. Screen Shot 2022-09-08 at 11.30.50 PM.png

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45.

a Among the angles labeled 1 through 5 in the figure at right, find two pairs of equal angles.

Screen Shot 2022-09-08 at 11.34.12 PM.png

b \boldsymbol{\angle 4+\angle 2+\angle 5= _________}

c Use parts (a) and (b) to explain why the sum of the angles of a triangle is 180

46.

a In the figure below, find θ, and justify your answer.

Screen Shot 2022-09-08 at 11.36.40 PM.png

b Write an algebraic expression for θ in the figure below.

Screen Shot 2022-09-08 at 11.37.22 PM.png

47. ABCD is a rectangle. The diagonals of a rectangle bisect each other. In the figure, AQD=130. Find the angles labeled 1 through 5 in order, and give a reason for each answer.

Screen Shot 2022-09-08 at 11.39.13 PM.png

48. A tangent meets the radius of a circle at a right angle. In the figure, AOB=140. Find the angles labeled 1 through 5 in order, and give a reason for each answer.

Screen Shot 2022-09-08 at 11.40.24 PM.png


This page titled 1.0: Angles and Triangles is shared under a GNU Free Documentation License 1.3 license and was authored, remixed, and/or curated by Katherine Yoshiwara via source content that was edited to the style and standards of the LibreTexts platform.

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