Perimeter Puzzles: Finding Missing Sides!

Mathematics Grades 4th Grade 3:31 Video
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Lesson Description

Learn how to find the perimeter of shapes, even when a side length is missing. We'll use addition and a bit of detective work!

Video Resource

Finding perimeter when a side length is missing | Math | 3rd grade | Khan Academy

Khan Academy

Duration: 3:31
Watch on YouTube

Key Concepts

  • Perimeter is the total distance around the outside of a shape.
  • Missing side lengths can be found using known side lengths and relationships.
  • Addition is used to combine all the side lengths to find the perimeter.

Learning Objectives

  • Students will be able to define perimeter.
  • Students will be able to identify missing side lengths in a figure by using other side lengths.
  • Students will be able to calculate the perimeter of a figure, including when a side length is missing.

Educator Instructions

  • Introduction (5 mins)
    Begin by asking students what they know about perimeter. Review that perimeter is the distance around a shape. Show them different shapes and ask if they can identify the perimeter. Introduce the concept of missing side lengths.
  • Video Viewing (7 mins)
    Play the Khan Academy video: "Finding perimeter when a side length is missing | Math | 3rd grade | Khan Academy." Instruct students to pay attention to how Lindsay finds the missing side length and calculates the perimeter.
  • Guided Practice (10 mins)
    Work through an example problem together on the board. Draw a polygon with one missing side length. Guide students to use the information provided by the known side lengths to deduce the length of the missing side. Then, guide them to add all the side lengths to find the perimeter. Emphasize writing the units (e.g., cm, inches).
  • Independent Practice (10 mins)
    Provide students with a worksheet containing problems similar to the one in the video. Have them work independently to find the missing side lengths and calculate the perimeters. Circulate to provide assistance as needed.
  • Wrap-up (3 mins)
    Review the key concepts of the lesson. Ask students what they learned about finding perimeter and what strategies they found helpful.

Interactive Exercises

  • Shape Scavenger Hunt
    Have students find rectangular objects in the classroom (desks, books, etc.). Have them measure the sides and calculate the perimeter. If they can't measure all sides, have them estimate the missing side and solve it.

Discussion Questions

  • What is perimeter?
  • How can you find a missing side length of a shape if you know the other side lengths?
  • Why is it important to include the units (e.g., cm, inches) when stating the perimeter?

Skills Developed

  • Problem-solving
  • Addition
  • Measurement
  • Spatial Reasoning

Multiple Choice Questions

Question 1:

What is perimeter?

Correct Answer: The distance around the outside of a shape

Question 2:

A rectangle has sides of 5 cm and 8 cm. What is the perimeter?

Correct Answer: 26 cm

Question 3:

A square has a side length of 6 inches. What is its perimeter?

Correct Answer: 24 inches

Question 4:

If you know all the sides of a shape except for one, how can you find the missing side if you know the perimeter?

Correct Answer: Subtract the sum of the known sides from the perimeter

Question 5:

A shape has sides of 3 cm, 4 cm, 5 cm, and a missing side. The perimeter is 15 cm. What is the length of the missing side?

Correct Answer: 3 cm

Question 6:

What unit do we use to measure perimeter?

Correct Answer: Inches

Question 7:

A shape is made up of two rectangles joined together. To find the perimeter, you should...

Correct Answer: Only measure the outside edges

Question 8:

A garden is shaped like a triangle with sides 7ft, 9ft, and 5ft. How much fencing will you need to surround it?

Correct Answer: 21ft

Question 9:

Which of these shows how to find the perimeter of a shape with sides 2cm, 3cm, 4cm, and 5cm?

Correct Answer: 2 + 3 + 4 + 5

Question 10:

If a shape has a perimeter of 20 inches and all four sides are the same length, how long is each side?

Correct Answer: 5 inches

Fill in the Blank Questions

Question 1:

The distance around the outside of a shape is called the ________.

Correct Answer: perimeter

Question 2:

To find the perimeter, you need to ________ all the side lengths.

Correct Answer: add

Question 3:

If a rectangle has a length of 10 cm and a width of 4 cm, its perimeter is ________ cm.

Correct Answer: 28

Question 4:

If a shape has a perimeter of 16 inches and three sides are 3 inches each, the fourth side is ________ inches.

Correct Answer: 7

Question 5:

A square has four equal ________.

Correct Answer: sides

Question 6:

You can ________ all of the sides to find the perimeter.

Correct Answer: add

Question 7:

Centimeters and inches are both units to measure ________.

Correct Answer: length

Question 8:

The perimeter of a shape is always measured in ________ units.

Correct Answer: length

Question 9:

If a side is missing, you can use the other sides and the total ________ to find its length.

Correct Answer: perimeter

Question 10:

If you are finding the perimeter of your classroom, you would use _______ as your unit of measurement.

Correct Answer: feet

Educational Standards

4.GM.3:[Standard] Determine elapsed time and convert between units of time. 4.GM.2:[Standard] Recognize and measure attributes in real-world and mathematical situations using various tools. 4.N.2.4:[Objective] Apply and analyze models to solve multi-step problems requiring the use of addition, subtraction, and multiplication of multi-digit whole numbers. Use various strategies, including the relationship between operations, the use of appropriate technology, and the context of the problem to assess the reasonableness of results.

Teaching Materials

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