How to Master Two-Column Proofs: A Step-by-Step Tutorial

• Statements: This column lists the given information, definitions, postulates, properties, and theorems in a logical sequence.
• Reasons: Here, you provide the justification or rationale for each statement, linking back to established truths.

• Start with the Given: Begin by listing the given facts from the problem.
• Use Definitions: Often, a definition can be applied immediately after a given fact.
• Apply Postulates and Theorems: Use established truths to deduce new facts.
• Conclude: End with the statement you were asked to prove.

• Always ensure each step logically follows the previous one.
• Familiarize yourself with key geometric postulates and definitions to streamline the justification process.
• Work backwards occasionally. Start from what you want to prove and see what you need to get there.

Examples

1. Proof for: If two angles are supplementary to the same angle, then they are congruent to each other.

Practice Questions:

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1. Given: \( AB = CD \), prove: \( CD = AB \) using a two-column proof.
2. Given: \( \angle X \) is complementary to \( \angle Y \) and \( \angle Y \) is complementary to \( \angle Z \). Prove: \( \angle X \cong \angle Z \) using a two-column proof.
3. Why is it essential to list every step in a two-column proof?

1. Statements	Reasons
   1. \( AB = CD \)	Given
   2. \( CD = AB \)	Symmetric Property of Equality

2. Statements	Reasons
   1. \( \angle X \) and \( \angle Y \) are complementary	Given
   2. \( \angle Y \) and \( \angle Z \) are complementary	Given
   3. \( m\angle X + m\angle Y = 90^\circ \)	Definition of Complementary Angles
   4. \( m\angle Y + m\angle Z = 90^\circ \)	Definition of Complementary Angles
   5. \( m\angle X = 90^\circ – m\angle Y \)	Algebraic Manipulation
   6. \( m\angle Z = 90^\circ – m\angle Y \)	Algebraic Manipulation
   7. \( m\angle X = m\angle Z \)	Transitive Property of Equality

3. Every step in a two-column proof ensures clarity, logical progression, and the ability to trace back to foundational truths or given information. It establishes a structured flow of reasoning, making complex arguments understandable.

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