# Lesson 12

### Lesson Purpose

The purpose of this lesson is for students to use gridded rectangles to learn that area is additive.

### Lesson Narrative

Students first decompose a rectangle into parts to see that area is additive. Then, students decompose a figure composed of rectangles into rectangles in any way that makes sense to them, find the area of those rectangles, and add them together to find the area. Students learn that parentheses are grouping symbols that can be used in expressions or equations, and use them to represent how they decomposed the figure into rectangles. The work of this lesson connects to previous work because students find the area of rectangles within the figure before adding to find the total area of the figure.

• Action and Expression

Activity 2: Find the Rectangles

### Learning Goals

Teacher Facing

• Find the area of figures composed of rectangles.
• Recognize that area is additive.

### Student Facing

• Let's find the area of figures made up of rectangles.

Building On

Building Towards

### Lesson Timeline

 Warm-up 10 min Activity 1 15 min Activity 2 20 min Lesson Synthesis 10 min Cool-down 5 min

### Teacher Reflection Questions

Which students came up with an unexpected strategy in today’s lesson? What are some ways you can be more open to the ideas of each and every student?

### Suggested Centers

• Five in a Row: Multiplication (3–5), Stage 2: Factors 1–9 (Addressing)
• Five in a Row: Addition and Subtraction (1–2), Stage 8: Add within 1,000 with Composing (Supporting)