cabt math 8 - angles and angle measurements
DESCRIPTION
Lecture on Angles for Grade 8 at CABTTRANSCRIPT
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The session shall begin shortly…
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LET’S START
!
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An angle consists of two different rays that have the same initial point. The rays are sides of the angles. The initial point is called the vertex.
Angles
vertex
sidesA
B
C
Angles & Angle Measurements
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Naming Angles
A
B
C
Notation: We denote an angle with three points and “” symbol.
Naming:
Use the three points, with the vertex always in the middle. The angle can also be named with just the vertex point as long as it is a “stand alone angle”.
The angle shown can be named as:
BAC, CAB, A
Angles & Angle Measurements
1
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Use the diagram below to answer the following questions.
a. Name the vertex. R
b. Name the sides of the angle.
c. Name the angle three different ways. SRT, TRS, R
R
S
TRS and RT
Check your understanding
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Measure of an Angle
A
B
C
The MEASURE of an angle is the amount of rotation a side makes with respect to the other side. The unit is DEGREES ()
Notation: We put the letter “m” in front of the angle name to denote its measure.
The measure of the angle shown can be written as:
mBAC, mCAB, mA
Angles & Angle Measurements
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Example: Here are some examples of angles and their degree measurements.
Measure of an Angle
Angles & Angle Measurements
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ProtractorA device used to measure angles.
Measure of an Angle
Angles & Angle Measurements
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Measure of an Angle
Angles & Angle Measurements
A
B
C
What is the measure of this angle?
ANSWER: 50
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Measure of an Angle
Angles & Angle Measurements
A
B
C
What is the measure of this angle?
ANSWER: 110
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Measure of an Angle
Angles & Angle Measurements
A
B
C
What is the measure of this angle?
ANSWER: 90
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Measure of an Angle
Angles & Angle Measurements
A
B
C
What is the measure of this angle?
ANSWER: 70
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Reading a Protractor
Date: 01/13/2014 at 15:51:52 From: Donya InaSubject: Reading a Protractor
Anak, alam kong may protractor ka at marunong kang mag-measure ng angle. At least may pakinabang ‘yan kesa kung anu-ano ang ipino-post mo sa FB. Pero tanong ko lang: which side of a protractor scale do you use?
Paki-explain. Lab U!
Angles & Angle Measurements
Measure of an Angle
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Date: 01/13/2014 at 15:52:53 From: Anak Subject: Re: Reading a Protractor
Donya Ina, I think that the easiest way to decide which scale to use is to look at the angle you're measuring. If it is greater than a right angle (more than ninety degrees) then use the scale that has numbers greater than 90. If the angle is obviously acute (measures less than 90 degrees), use the scale which has numbers less than 90. Of course you have to line up the protractor correctly with the center point on the vertex of the angle and one of the rays of the angle coincide with the line on the protractor.
I-push mo lang ‘yan!
Angles & Angle Measurements
Measure of an Angle
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Wala Lang…
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Two angles are congruent if they have the same measure.Symbol: “” (is congruent to)
Congruent Angles
B
A
C
Angles & Angle Measurements
Y
X
Z
ABC and XYZ are congruent, so we write
ABC XYZ which means mABC = mXYZ
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Straight angle
m A= 180°
Obtuse angle
90 < m A < 180°
Acute angle
0< m A < 90°
Classifying Angles
Angles are classified as acute, right, obtuse, and straight, according to their measures. Angles have measures greater than 0°and less or equal to 180°.
A ARight angle
m A = 90°
A A
Angles & Angle Measurements
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An acute angle is an angle measuring between 0 and 90 degrees.
Classifying Angles
Angles & Angle Measurements
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An obtuse angle is an angle measuring between 90 and 180 degrees.
Classifying Angles
Angles & Angle Measurements
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Classifying Angles
Angles & Angle Measurements
A right angle is an angle measuring exactly 90 degrees. It is formed by two perpendicular rays with a common endpoint. Denotes 90
degrees
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Classifying Angles
Angles & Angle Measurements
A straight angle is an angle measuring exactly 180 degrees. It is formed by two opposite rays with a common
endpoint.
A
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Determine whether each of the following is acute, right, or obtuse.
1.
2.
3.
4.
5.
??
acute obtuseright
obtuseacute
Check your understanding
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Complementary AnglesTwo angles are called complementary angles if the sum of their degree measurements equals 90 degrees.We say that one is the COMPLEMENT of the other.
Example: These two angles are complementary.
Angle Pairs
Angles & Angle Measurements
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These two angles can be "pasted"
together to form a right angle!
Angle Pairs
Angles & Angle Measurements
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Supplementary AnglesTwo angles are called supplementary angles if the sumof their degree measurements equals 180 degrees.
We say that one is the SUPPLEMENT of the other.
Example: These two angles are supplementary.
Angle Pairs
Angles & Angle Measurements
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These two angles can be "pasted" together to
form a straight line!
Angle Pairs
Angles & Angle Measurements
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1. Find the complement of 65.
Examples. (See solution on the board)
Answer : 25
Answer : 40
Angle Pairs
Angles & Angle Measurements
2. Two angles are supplementary. One measures 140. What is the measure of the other angle?
Answer : 30
3. The supplement of an angle is 30 more than twice its complement. What is the angle?
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A. Determine the complement of
1. 48 2. 7942
Check your understanding
11
B. Determine the supplement of
1. 48 2. 123132 57
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Angle Pairs
Angles & Angle Measurements
Adjacent Angles
Adjacent angles are two angles with a common vertex and a common side but no common interior points.
A
B
C
DAdjacent angles:
BAC and DAB
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Angle Pairs
Angles & Angle Measurements
Angle Addition PostulateThe sum of the measures of two adjacent angles is equal to the measure of the angle formed by the two nonadjacent rays.
A
B
C
D mBAC + mDAB
= mDAC
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Angle Pairs
Angles & Angle Measurements
Linear PairsTwo adjacent angles form a linear pair if their uncommon sides form opposite rays
B
D
C
AABD and mDBC are linear pairs
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Angle Pairs
Angles & Angle Measurements
Linear Pair PostulateLinear pairs are supplementary.
B
D
C
A
Since ABD and mDBC are linear pairs, then
they are supplementary; that
is, mABD + mDBC
= 180
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Examples: Find x (see solution on the board).
2. 3.x
55 165x
x =35 x=15
Angle Pairs
Angles & Angle Measurements
1.
55x
P
U
S
H mPUH =140
x = 85
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Examples:
Angle Pairs
Angles & Angle Measurements
3. The angles (3x – 20) and (7x) form a linear pair. What is the a. value of x? b. measure of each angle?
x = 20
40 and 140
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Vertical Angles
VERTICAL angles are two nonadjacent angles formed by two intersecting lines.
Angle Pairs
Angles & Angle Measurements
AED and BEC are vertical angles
AEB and DEC are vertical angles
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Vertical Angle TheoremVertical angles are congruent
Angle Pairs
Angles & Angle Measurements
AED and BEC are vertical angles, so AED BEC or mAED = mBEC
AEB and DEC are also vertical angles, so AEB DEC or mAEB = mDEC
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Vertical AnglesNote that the adjacent angles form linear pairs.
Angle Pairs
Angles & Angle Measurements
The ff. are linear pairs:
AEB and BEC
AED and AEB
AED and CED
DEC and BEC
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Example
1.
Angle Pairs
Angles & Angle Measurements
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Example:
2. Find the value of x, y, and z.
58x
x = 58y = 122x = 122y
z
Angle Pairs
Angles & Angle Measurements
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20
C
J
D
E F
G
H
70
9070
20
90
3. Given the figure shown on the right. Find the measures of the following:
)a DCE)b GCH)c ECF)d DCJ)e JCH
Angle Pairs
Angles & Angle Measurements
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52
B
A
F E
D
C
60
G
68
68
6052
Check your understandingFind the measures of the numbered angles.
3 2
14
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Summing it up!Angles & Angle Measurements
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Thank you!