1. Lesson 2-5: Algebraic Proofs
Rigor – Use properties of equality,
postulates, and definitions to justify
steps in a proof.
Relevance – logical thinking and
proofs

2. Notes Highlight the properties on pg71 of the corebook
Add the distributive property of equality to the list
Distributive Property of Equality:
a(b + c) = ab + bc

3. Example: Justifying single statements
Symmetric Property of Congruence
Distributive Property of Equality

4. Example: Justify each step used to solve for x.
Think, what relationship do you see in the diagram?
Definition: supplementary angles
Substitution Property
Combine like terms
Subtraction property of equality
Division property of equality

5. Example 2: Complete the proof
Definition of a bisector
Substitution Property
Subtraction property of equality
Division property of equality

6. Two – Column Proofs Format
Statements
First statement is always given info
This side is for work
No skipping steps
Last line is ALWAYS what you are asked to find/prove
Reasons
First reason is ALWAYS GIVEN info (picture, words, definitions)
Definitions, theorems, properties, formulas go here
The last reason is NEVER “prove”

7. Back to the core book Turn to page 72
We will do practice problems 1 and 2 together

8. 2-5 Classwork from the Core Book
Pg 73 #4 – 8
Be ready to move on in 15 minutes
You will be assigned 2-5 homework next week