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2.4: Algebraic and Congruence Properties

Difficulty Level: At Grade Created by: CK-12

Learning Objectives

  • Understand basic properties of equality and congruence.
  • Solve equations and justify each step in the solution.
  • Use a 2-column format to prove theorems.

Review Queue

Solve the following problems.

1. Explain how you would solve .

2. If two angles are a linear pair, they are supplementary.

If two angles are supplementary, their sum is .

What can you conclude? By which law?

3. Draw a picture with the following:

Know What? Three identical triplets are sitting next to each other. The oldest is Sara and she always tells the truth. The next oldest is Sue and she always lies. Sally is the youngest of the three. She sometimes lies and sometimes tells the truth.

Scott came over one day and didn't know who was who, so he asked each of them one question. Scott asked the sister that was sitting on the left, “Which sister is in the middle?” and the answer he received was, “That's Sara.” Scott then asked the sister in the middle, “What is your name?” The response given was, “I'm Sally.” Scott turned to the sister on the right and asked, “Who is in the middle?” The sister then replied, “She is Sue.” Who was who?

Properties of Equality

Recall from Chapter 1 that the = sign and the word “equality” are used with numbers.

The basic properties of equality were introduced to you in Algebra I. Here they are again:

For all real numbers , and :

Examples
Reflexive Property of Equality
Symmetric Property of Equality and or
Transitive Property of Equality and , then and , then
Substitution Property of Equality If , then can be used in place of and vise versa. If and , then
Addition Property of Equality If , then . If , then
Subtraction Property of Equality If , then . If , then
Multiplication Property of Equality If , then . If , then
Division Property of Equality If , then . If , then
Distributive Property

Properties of Congruence

Recall that if and only if . and represent segments, while and are lengths of those segments, which means that and are numbers. The properties of equality apply to and .

This also holds true for angles and their measures. if and only if . Therefore, the properties of equality apply to and .

Just like the properties of equality, there are properties of congruence. These properties hold for figures and shapes.

For Line Segments For Angles
Reflexive Property of Congruence
Symmetric Property of Congruence If , then If , then
Transitive Property of Congruence If and , then If and , then

Using Properties of Equality with Equations

When you solve equations in algebra you use properties of equality. You might not write out the logical justification for each step in your solution, but you should know that there is an equality property that justifies that step. We will abbreviate “Property of Equality” “PoE” and “Property of Congruence” “PoC.”

Example 1: Solve and justify each step.

Solution:

Example 2: Given points , and , with , and . Are , and collinear?

Solution: Set up an equation using the Segment Addition Postulate.

Because the two sides are not equal, and are not collinear.

Example 3: If and , prove that is an acute angle.

Solution: We will use a 2-column format, with statements in one column and their corresponding reasons in the next. This is formally called a 2-column proof.

Statement Reason
1. and Given (always the reason for using facts that are told to us in the problem)
2. Substitution PoE
3. Subtraction PoE
4. is an acute angle Definition of an acute angle,

Two-Column Proof

Example 4: Write a two-column proof for the following:

If , and are points on a line, in the given order, and , then .

Solution: First of all, when the statement is given in this way, the “if” part is the given and the “then” part is what we are trying to prove.

Always start with drawing a picture of what you are given.

Plot the points in the order on a line.

Add the corresponding markings, , to the line.

Draw the 2-column proof and start with the given information. From there, we can use deductive reasoning to reach the next statement and what we want to prove. Reasons will be definitions, postulates, properties and previously proven theorems.

Statement Reason
1. , and are collinear, in that order. Given
2. Given
3. Reflexive PoE
4. Addition PoE
5. Segment Addition Postulate
6. Substitution or Transitive PoE

When you reach what it is that you wanted to prove, you are done.

Prove Move: (A subsection that will help you with proofs throughout the book.) When completing a proof, a few things to keep in mind:

  • Number each step.
  • Start with the given information.
  • Statements with the same reason can (or cannot) be combined into one step. It is up to you. For example, steps 1 and 2 above could have been one step. And, in step 5, the two statements could have been written separately.
  • Draw a picture and mark it with the given information.
  • You must have a reason for EVERY statement.
  • The order of the statements in the proof is not fixed. For example, steps 3, 4, and 5 could have been interchanged and it would still make sense.

Example 5: Write a two-column proof.

Given: bisects ;

Prove:

Solution: First, put the appropriate markings on the picture. Recall, that bisect means “to cut in half.” Therefore, if bisects , then . Also, because the word “bisect” was used in the given, the definition will probably be used in the proof.

Statement Reason
1. bisects Given
2. Definition of an Angle Bisector
3. If angles are , then their measures are equal.
4. Angle Addition Postulate
5. Substitution PoE
6. Substitution PoE
7. Subtraction PoE
8. If measures are equal, the angles are .

Prove Move: Use symbols and abbreviations for words within proofs. For example, was used in place of the word congruent above. You could also use for the word angle.

Know What? Revisited The sisters, in order are: Sally, Sue, Sara. The sister on the left couldn’t have been Sara because that sister lied. The middle one could not be Sara for the same reason. So, the sister on the right must be Sara, which means she told Scott the truth and Sue is in the middle, leaving Sally to be the sister on the left.

Review Questions

For questions 1-8, solve each equation and justify each step.

For questions 9-14, use the given property or properties of equality to fill in the blank. , and are real numbers.

  1. Symmetric: If , then _________.
  2. Distributive: If , then _________.
  3. Transitive: If and , then _________.
  4. Symmetric: If , then _________.
  5. Transitive: If and , then _________.
  6. Substitution: If and , then _________.
  7. Given points , and and , and . Determine if and are collinear.
  8. Given points and and and . Are the three points collinear? Is the midpoint?
  9. If and , explain how must be an obtuse angle.

Fill in the blanks in the proofs below.

  1. Given: Prove:
Statement Reason
1. Given
2.
3. Addition PoE
4.
  1. Given: is the midpoint of . is the midpoint Prove:
Statement Reason
1. Given
2. Definition of a midpoint
3.

Use the diagram to answer questions 20-25.

  1. Name a right angle.
  2. Name two perpendicular lines.
  3. Given that , is true? Explain your answer.
  4. Is a right angle? Why or why not?
  5. Using what is given in the picture AND , prove . Write a two-column proof.
  6. Using what is given in the picture AND , prove . Write a two-column proof.

Use the diagram to answer questions 26-32.

Which of the following must be true from the diagram?

Take each question separately, they do not build upon each other.

  1. is a square
  2. bisects
  3. Write a two-column proof. Given: Picture above and bisects Prove:
  4. Draw a picture and write a two-column proof. Given: and form a linear pair and . Prove: is a right angle

Review Queue Answers

  1. First, subtract 3 from both sides and then divide both sides by 2.
  2. If 2 angles are a linear pair, then their sum is . Law of Syllogism.

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