Geometry 2_4 and 2_5 Updated Version
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Transcript Geometry 2_4 and 2_5 Updated Version
GEOMETRY: CHAPTER 2
Ch. 2.4 Reason Using
Properties from Algebra
Ch. 2.5: Prove Statements
about Segments and
Angles
2.4: KEY CONCEPT:
Algebraic Properties of Equality
Let a, b, and c be real numbers.
Addition Property
If a=b, then a + c = b + c.
Subtraction Property
If a=b, then a - c = b – c.
Multiplication Property
If a=b, then ac = bc.
Division Property
If a=b and c≠0, then
a/c=b/c.
Substitution Property:
If a=b, then a can be substituted for b in
any equation or expression.
Distributive Property:
a (b + c ) = ab + ac.
Ex. 1 Solve: Write reasons for each step
2x + 4 = 10 – 4x
Write original
equation.
Given
2x +4 +4x = 10 - 4x+4x Add 4x to each Addition Property
side.
of Equality
6x + 4 = 10
Combine Like
terms.
Simplify.
6x = 6
Subtract four
Subtraction
from each side. Property of Equality
x=1
Divide each
side by 6.
Division Property
of Equality
Ex. 2: Use the Distributive Property and
Write reasons for each step.
Solution:
Equation
Explanation
Reason
-2 (3x + 1)=40 Write original equation Given
-6x -2 = 40
Multiply
Distribution
Property
-6x = 42
Add 2 to each side
Addition
Property of
Equality
x= -7
Divide each side by -6 Division
Property of
Equality
Key Concepts:
Reflexive Property of Equality:
Real Numbers: for any real number a, a=a.
Segment Length: for any segment AB,
AB=AB.
Angle Measure: for any angle A, mA mA.
Symmetric Property of Equality
Real Numbers:
For any real numbers a and b, if a b, then b a.
Segment Length:
For any segments AB and CD, if AB CD, then CD AB.
Angle Measure:
For any angles A and B, if mA mB, then mB mA.
Transitive Property of Equality
Real Numbers:
For any real numbers a, b and c, if a b and b c, then a c.
Segment Length:
For any segments AB, CD and EF , if AB CD and CD EF ,
then AB EF .
Angle Measure:
For any angles A, B and C , if mA mB and mB mC ,
then mA mC.
Ex. 3: The city is planning to add two stations
between the beginning and end of a
commuter train line. Use the information
given. Determine whether RS=TU.
Image taken from: Geometry. McDougal Littell: Boston, 2007. P. 107.
Given: RT=SU
Prove: RS=TU
EQUATION
RT=SU
ST=ST
RT-ST=SU-ST
RT-ST=RS
SU-ST=TU
RS=TU
Ex. 3 (cont.)
REASON
Given
Reflexive Property
Subtraction Property of Equality
Segment Addition Postulate
Segment Addition Postulate
Substitution Property of Equality
For more examples, go to CH. 2, Lesson 5,
Example 4
http://www.classzone.com/cz/books/geometry_2007_na/get_chapter_group.htm?cin=2
&rg=help_with_the_math&at=powerpoint_presentations&var=powerpoint_presentation
s
2.5 Prove Statements about Segments and Angles
A proof is a logical argument that
shows a statement is true.
A two-column proof has numbered
statements and corresponding reasons
that show an argument in a logical
order.
In a two-column proof, each statement
in the left-hand column is either
given information or the result of
applying a known property or fact to
statements already made.
The explanation for the corresponding
statement is in the right-hand
column.
Ex. 5
Image taken from: Geometry. McDougal Littell: Boston, 2007. P. 112.
You are designing
a logo to sell daffodils. Use the information given.
Determine whether the measure of angle EBA is equal
to the measure of angle DBC.
Image taken from: Geometry. McDougal Littell: Boston, 2007. P. 112.
Ex. 5 (cont.)
STATEMENTS
1. m1 m3
REASON
Given
2. mEBA m3 m2 Angle Addition Postulate
3. mEBA m1 m2 Substitution Property of Equality
4. m1 m2 mDBC Angle Additon Postulate
5. mEBA mDBC
Transitive Property of Equality
Theorems—The reasons used in a
proof can include definitions,
properties, postulates, and theorems.
A theorem is a statement that can be
proven. Once you have proven a
theorem, you can use the theorem as
a reason in other proofs.
Theorem 2.1 Congruence of Segments.
Segment congruence is reflexive,
symmetric, and transitive.
Re flexive: For any segments AB, if AB AB.
Symmetric: If AB CD, then CD AB.
Transitive: If AB CD and CD EF , then AB EF .
Theorem 2.2 Congruence of Angles
Angle congruence is reflexive, symmetric, and
transitive.
Reflexive: For any angle A, mA mA.
Symmetric: If A B, then B A.
Transitive: If A B and B C , then A C.
Ex. 6: Name the property illustrated by
the statement.
a. If K L and L M , then K M .
b. If EF GH , then GH EF .
a. Transitive Property of Angle
Congruence.
b. Symmetric Property of Segment
Congruence.
Ex. 7 Use properties of equality
Prove this property of midpoints: If you know that M
is the midpoint of segment AB, prove that AB is two
times AM and AM is one half of AB.
GIVEN: M is the midpoint of segment AB.
Prove: a. AB=2 (AM)
b. AM= ½ AB
Ex. 7 (cont.)
STATEMENTS
REASONS
1. M is the midpoint of AB. 1. Given
2. AM . MB.
2. Definition of a Midpoint
3. AM MB.
3. Def. of congruent segments
4. AM MB AB.
4. Segment Addition Postulate
5. AM AM AB.
5. Substitution Property of Equality
a. 6. 2 AM AB
6. Distributive Property
b. 7. AM 1 AB
2
7. Division Property of Equality
Turn to page 103 in your book and look at the
bottom right example. Refer back to this page for
writing proofs!
Ex. 8
Go To Chapter 6, Lesson 2, Example 4:
http://www.classzone.com/cz/books/geometry_2
007_na/get_chapter_group.htm?cin=2&rg=hel
p_with_the_math&at=powerpoint_presentation
s&var=powerpoint_presentations
Write this example in your notes as Ex. 8.