# Lesson 21

Odd and Even Numbers

These materials, when encountered before Algebra 1, Unit 7, Lesson 21 support success in that lesson.

### Lesson Narrative

In this lesson, students practice examining a proof by contradiction using sums of even and odd numbers. First, students examine some concrete examples of sums and products of even and odd numbers to evaluate whether some claims are always, sometimes, or never true. Next, students follow a proof by contradiction that shows that the sum of an even and odd number must always be even.

In the associated Algebra 1 lesson, students examine a similar proof by contradiction about sums and products of rational and irrational numbers. They are supported by following a similar process with more familiar types of numbers. Students reason abstractly and quantitatively (MP2) when they think of specific examples to prove or disprove a statement.

### Learning Goals

Teacher Facing

- Use properties of even and odd numbers to reason about their sums and products.

### Student Facing

- Let’s explore even and odd numbers.

### CCSS Standards

Building Towards