Lesson 8 Find the Perimeter

    • Let’s find the perimeter of more shapes.

Warm-up Number Talk: Decreasing Dividend

Find the value of each expression mentally.

Activity 1 Ways to Find Perimeter

Problem 1

What do you notice? What you wonder?

Diagram. 2 rectangles. One on a 5 by 3 dot grid, one with sides marked 5 and 3.

Problem 2

Find the perimeter of each shape. Explain or show your reasoning.

  1.  
    Triangle with sides marked 10, 8, and 12 centimeters.
  2.  
    Rectangle with sides marked 16 and 24 inches.
  3.  
    5-sided shape with sides marked 13, 5, 5, 11, and 2 centimeters.
  4.  
    Hexagon with 2 long sides, 8 feet, and 4 short sides, 4 feet.
  5.  
    Shape with four sides that are 9 centimeters.

Activity 2 Something is Missing

  1. Find the perimeter of this rectangle. Explain or show your reasoning.

    Rectangle with sides marked 21 and 9 centimeters.

  2. All the short sides of this figure are the same length, and all the angles are right angles. Find the perimeter. Explain or show your reasoning.

    Figure with 12 sides. 11 short sides, each 40 units long. 1 long side of unknown length.

  3. All the sides of the octagon are the same length. Find the perimeter. Explain or show your reasoning.

    Octagon. Each side 30 inches.

Practice Problem

Problem 1

Find the perimeter of each shape. Explain or show your reasoning.

  1.  

    Diagram. Rectangle with two sides 11 centimeters and two sides 6 centimeters.

  2.  

    Diagram. Parallelogram with two sides 12 centimeters and two sides 5 centimeters.