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Angles

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Angles and Lines
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Angles are formed by intersecting lines or rays. If you take any two lines or rays, will you form at least one angle?

Watch This

http://www.youtube.com/watch?v=7iBc5bJdanI James Sousa: Angle Basics

Guidance

A line segment is a portion of a line with two endpoints. A ray is a portion of a line with one endpoint. Line segments are named by their endpoints and rays are named by their endpoint and another point. In each case, a segment or ray symbol is written above the points. Below, the line segment is \overline{AB} and the ray is \overrightarrow{CD} .

When two rays meet at their endpoints, they form an angle . Depending on the situation, an angle can be named with an angle symbol and by its vertex or by three letters. If three letters are used, the middle letter should be the vertex. The angle below could be called \angle B or \angle ABC or \angle CBA . Use three letters to name an angle if using one letter would not make it clear what angle you are talking about.

Angles are measured in degrees. You can use a protractor or geometry software to measure angles. Remember that a full circle has 360^\circ .

 

An angle that is exactly 90^\circ (one quarter of a circle) is called a right angle . A right angle is noted with a little square at its vertex. An angle that is more than 90^\circ but less than 180^\circ is called an obtuse angle . An angle that is less than 90^\circ   is called an acute angle . An angle that is exactly 180^\circ (one half of a circle) is called a straight angle .

 

Two angles are complementary if the sum of their measures is 90^\circ . Two angles are supplementary if the sum of their measures is  180^\circ . Two angles that together form a straight angle will always be supplementary. When two lines intersect, many angles are formed, as shown below.

In the diagram above \angle AEC and \angle AED are adjacent angles because they are next to each other and share a ray. They are also supplementary because together they form a straight angle. \angle AEC and \angle DEB are called vertical angles . You can show that vertical angles will always have the same measure.

Example A

Explain why you must use three letters to identify any of the angles in the diagram below.

Solution : All angles in this diagram have a vertex of E . Therefore, \angle E is ambiguous because it could refer to many different angles. Use three letters with  E as the middle letter to be clear about which angle you are referring to.

Example B

\angle ABC and \angle DEF are complementary angles with m\angle ABC =40^\circ . What is m\angle DEF ?

Solution: The “ m ” in front of the angle symbol is read as “the measure of”. m\angle ABC means “the measure of angle ABC ”. Because the two angles are complementary, their measures must add to 90^\circ . Therefore, m\angle DEF =50^\circ .

Example C

Let m\angle AEC=x^\circ . Show that m\angle DEB must also equal x^\circ .

Solution : If m\angle AEC=x^\circ , then m\angle AED=(180-x)^\circ because \angle AEC and \angle AED form a straight angle and are therefore supplementary. Similarly, m\angle DEB=[180-(180-x) ]^\circ=(180-180+x)^\circ=x^\circ . This is how you can be confident that vertical angles will always have the same measure.

Concept Problem Revisited

As long as the lines or rays intersect, at least one angle will be formed. If the lines (or rays) are parallel , and therefore don't intersect , then no angles will be formed.

 

Vocabulary

A line segment is a portion of a line with two endpoints.

A ray is a portion of a line with one endpoint.

When two rays meet at their endpoints, they form an angle .

An angle that is exactly 90^\circ (one quarter of a circle) is called a right angle .

An angle that is more than 90^\circ but less than  180^\circ is called an obtuse angle .

An angle that is less than 90^\circ is called an acute angle .

An angle that is exactly 180^\circ (one half of a circle) is called a straight angle .

Two angles are complementary if the sum of their measures is  90^\circ .

Two angles are supplementary if the sum of their measures is 180^\circ .

When two lines intersect, adjacent angles are next to each other and share a ray. Vertical angles are across from one another and only share a vertex.

Guided Practice

1. Name the angle below and classify it by its size.

 

2. Estimate the measure of the angle from #1. Use a protractor to confirm your answer.

3. What are two lines that form at a right angle called?

Answers:

1. \angle F or \angle DFE or \angle EFD . It is an acute angle.

2. Remember that exactly half of a right angle is 45^\circ . This angle looks to be more than half of a right angle. You might guess that it is approximately 55^\circ . Using a protractor, you can see that it is about 60^\circ .

 

3. Perpendicular lines.

Practice

1. What's the difference between a line segment, a line, and a ray?

2. Draw an example of a right angle.

3. Draw an example of an obtuse angle.

4. Draw an example of an acute angle.

5. Why are two angles that make a straight angle always supplementary?

6. If m\angle ABC=(2x+4)^\circ , m\angle DEF=(3x-5)^\circ , and \angle ABC and \angle DEF are complementary, what are the measures of the angles?

7. If m\angle ABC=(2x+4)^\circ , m\angle DEF=(3x-5)^\circ , and \angle ABC and \angle DEF are supplementary, what are the measures of the angles?

Use the diagram below for #8-#12.

 

8. Give an example of vertical angles.

9. Give an example of a straight angle.

10. Give an example of supplementary angles.

11. If m\angle ABC=70^\circ , find m\angle ABF .

12. If m\angle ABC=70^\circ , find m\angle FBG .

13. What do you remember about perpendicular lines?

Use the angle below for #14-#15.

 

14. Name the angle and classify it based on its size.

15. Estimate the measure of the angle. Use a protractor to confirm your answer.

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