Slason-Math

Formula for the Area of a Triangle

Grade Level: Four

Approximate Time: 60 minutes

Goal: Demonstrates conceptual understanding of perimeter of polygons, and the area of rectangles, polygons or irregular shapes on grids using a variety of models, manipulatives, or formulas. Express all measures using appropriate units.

Objectives: 1.1: Students will demonstrate a conceptual understanding of the area of triangles. 1.2: Students will demonstrate an understanding of the formula for the area of a triangle. 1.3: Students will demonstrate an understanding of using appropriate units of measurement.

Materials and Resources: //-Math Journal// 2 -Teaching Master -Centimeter ruler -Scissors and transparent tape -Square-corner device
 * Only hard copies are available for the previous two materials**

Pre-assessment

Introduction: I will begin by asking the students the formulas for area of a rectangle and parallelogram to generate prior knowledge. Then I will ask the students what they know about triangles. We will have a quick discussion and make a list on the board of their responses. Lesson Development: 1. I will begin by drawing a triangle on the board. I will label one side (the side that “sits”) the **base**. Then I will explain to the class that the //base// is also used to mean //length of base//. 2. Next, I will draw a dashed line to show the **height** (also label and include a right-angle symbol). Explain that the height is the shortest distance from the vertex above the base to the base. 3. I will have the class open their math journal to page 254. Then I will hand out //Math Masters//, p.123 to the class and point out that Triangle A and B on the master are the same as Triangle A on the journal. 4. Then I will lead the students in the following activity: -Cut out Triangles A and B from the master. -Tape the triangles together at the shaded corners to form a parallelogram. (Partners should be working together) -Tape the parallelogram in the space next to Triangle A in the journal. -Record the dimensions and area of the triangle and the area of the parallelogram. -Have the students repeat steps with Triangles C and D, E and F, and G and H. (can work with a partner of the people around them)** Closure: If the length of the base and the height of a triangle are the same as the length of the base and the height of a parallelogram, then: Area of the triangle=½ the Area of the parallelogram Using variables: A=½ of (b*h) or A=½ *(b*h) Where //b// is the length of the base and //h// is the height.** Have students record this formula and the bottom of their journal.** 6. Have students turn to page 256 in their math journal. Lead the students, and work on question 6 as a class. 7. Have students work with a partner or classmates near them to finish question 7 and 8. 8. Bring the class together: go over the answers to questions 6-8. Ask for volunteers to share their responses. Discuss the answers. 9. Ask if the students have any questions. Go over these with the class. 10. Tell students that they will be assigned the Study Link 8.7 assignment for homework as well as fill out a sheet reflecting on the day’s lesson on finding area of triangles.
 * At this point I will lead the class to discuss the relationship between the area of the triangle and the area of the parallelogram. Triangles A and B have the same area. Therefore, the area of either triangle is half the area of the parallelogram.
 * Base of the triangle and of parallelogram= 6cm
 * Height of triangle and of parallelogram= 4cm
 * Area of parallelogram=24cm2
 * Area of triangle=½ the area of parallelogram=12cm2
 * 5. I will bring the class together and ask the students to state a rule and write a formula for the area of a triangle.
 * The rule can be written in these ways:

Assessment: Anecdotal/Affective

Learner Factors: -This lesson allows for students to use hands-on materials to visualize how to calculate the area of triangles. This is beneficial for students who don’t learn as well from just hearing and doing standard worksheets. -Students will be working as a large group, as well as in pairs or small groups throughout the lesson. This way the students can work together to come to conclusions about finding area. If they have questions they have the opportunity to work through it with peers as well as getting assistance from the teacher. -If a student finishes early, there will be other worksheets for the student to continue practice with finding area. -I will be available for students, walking around the room, answering any questions or misconceptions they may have about finding area.

Environment: -Students will begin the lesson by brainstorming all of the things that they know about triangles to trigger prior knowledge and get them thinking about what is to come in the lesson. -Students will be working as a large group, as well as in small groups or pairs throughout the entire lesson.

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