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Two-Column Proofs

Geometric proofs are among the building blocks of math. Learn how to use two column proofs to assert and prove the validity of a statement.

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Two-Column Proofs

What if you wanted to prove that two angles are congruent? After completing this Concept, you'll be able to formally prove geometric ideas with two-column proofs.

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CK-12 Foundation: Chapter2TwoColumnProofsA

James Sousa: Introduction to Proof Using Properties of Congruence

Guidance

A two-column proof is one common way to organize a proof in geometry. Two-column proofs always have two columns- statements and reasons. The best way to understand two-column proofs is to read through examples.

When writing your own two-column proof, keep these things in mind:

  • Number each step.
  • Start with the given information.
  • Statements with the same reason can be combined into one step. It is up to you.
  • 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 always fixed, but make sure the order makes logical sense.
  • Reasons will be definitions, postulates, properties and previously proven theorems. “Given” is only used as a reason if the information in the statement column was told in the problem.
  • Use symbols and abbreviations for words within proofs. For example, can be used in place of the word congruent. You could also use for the word angle.

Example A

Write a two-column proof for the following:

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

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 two-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. 1. Given
2. 2. Given
3. 3. Reflexive PoE
4. 4. Addition PoE
5. 5. Segment Addition Postulate
6. 6. Substitution or Transitive PoE

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

Example B

Write a two-column proof.

Given: bisects ;

Prove:

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

Example C

The Right Angle Theorem states that if two angles are right angles, then the angles are congruent. Prove this theorem.

To prove this theorem, set up your own drawing and name some angles so that you have specific angles to talk about.

Given: and are right angles

Prove:

Statement Reason
1. and are right angles 1. Given
2. and 2. Definition of right angles
3. 3. Transitive
4. 4. angles have = measures

Any time right angles are mentioned in a proof, you will need to use this theorem to say the angles are congruent.

Example D

The Same Angle Supplements Theorem states that if two angles are supplementary to the same angle then the two angles are congruent. Prove this theorem.

Given: and are supplementary angles. and are supplementary angles.

Prove:

Statement Reason

1. and are supplementary

and are supplementary

1. Given

2.

2. Definition of supplementary angles
3. 3. Substitution
4. 4. Subtraction
5. 5. angles have = measures

Example E

The Vertical Angles Theorem states that vertical angles are congruent. Prove this theorem.

Given: Lines and intersect.

Prove:

Statement Reason
1. Lines and intersect 1. Given

2. and are a linear pair

and are a linear pair

2. Definition of a Linear Pair

3. and are supplementary

and are supplementary

3. Linear Pair Postulate

4.

4. Definition of Supplementary Angles
5. 5. Substitution
6. 6. Subtraction
7. 7. angles have = measures

Watch this video for help with the Examples above.

CK-12 Foundation: Chapter2TwoColumnProofsB

Vocabulary

A two-column proof is one common way to organize a proof in geometry. Two-column proofs always have two columns: statements and reasons.

Guided Practice

1. Write a two-column proof for the following:

Given: and

Prove:

2. Write a two-column proof for the following:

Given: is supplementary to , is supplementary to ,

Prove:

Answers:

1.

Statement Reason
1. 1. Given
2. and are right angles 2. Definition of a Perpendicular Lines
3. 3. Right Angle Theorem
4. 4. Given
5. 5. Subtraction

2.

Statement Reason
1. is supplementary to , is supplementary to 1. Given
2. , 2. Definition of Supplementary Angles
3. 3. Substitution
4. 4. Given
5. 5. Substitution
6. 6. Subtraction

Practice

Write a two-column proof for questions 1-5.

  1. Given: Prove:
  2. Given: and Prove:
  3. Given: Prove:
  4. Given: Prove:
  5. Given: Prove: and are complements

Use the picture for questions 6-15.

Given: is the midpoint of and

is the midpoint of

is the midpoint of

  1. List two pairs of vertical angles.
  2. List all the pairs of congruent segments.
  3. List two linear pairs that do not have as the vertex.
  4. List a right angle.
  5. List two pairs of adjacent angles that are NOT linear pairs.
  6. What is the perpendicular bisector of ?
  7. List two bisectors of .
  8. List a pair of complementary angles.
  9. If is an angle bisector of , what two angles are congruent?
  10. Fill in the blanks for the proof below.

Given: Picture above and

Prove: is the angle bisector of

Statement Reason

1.

is on the interior of

1.
2. 2.
3. 3. Angle Addition Postulate
4. 4. Substitution
5. 5.
6. 6. Division PoE
7. 7.

Vocabulary

linear pair

linear pair

Two angles form a linear pair if they are supplementary and adjacent.

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