Unit 4
A C T I V I T Y 24
AC TIVIT Y
25 MIN
Perimeter
CLASS
Give each student a sheet of centimeter grid paper. Today, we will find the distance
around different figures. The distance around a figure is called its perimeter.
Use after
Unit 4, Session 4.7
Vocabulary/Vocabulario
perimeter/
perímetro
formula/
fórmula
Draw a 4-by-6 rectangle on your grid paper.
Help students use the small squares in the grid to find the
perimeter of the rectangle. What is the perimeter?
Explain that since each side of a small square measures
1 centimeter, the perimeter is 20 centimeters.
Can you think of another way to find the perimeter?
Students might say:
Materials/Materiales
“We can add: 6 ⴙ 4 ⴙ 6 ⴙ 4 ⴝ 20.”
“Because there are 2 lengths and 2 widths, we could
multiply 2 times 6 and 2 times 4. We could then add
12 and 8 to get 20.”
• centimeter grid paper/
papel cuadriculado a 1 centímetro
• Activity 24 Master (see below)/
Actividad 24 (ver abajo)
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Un i t 4
Activity 24
MASTER
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Size, Shape, and Symmetry
NOTE Students find the perimeter of
rectangles, squares, and combinations of
rectangles and squares.
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© Pearson Education, Inc. 4
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Suppose we let the letter P stand for perimeter, the letter ᐍ stand for length, and
the letter w stand for width. Since there are 2 lengths and 2 widths, we could write
the following number sentence: P ⴝ 2 ⴛ ᐍ ⴙ 2 ⴛ w. This is a special kind of
number sentence called a formula. Write the formula on the board, and have students
follow your example. The formula we wrote can be used to find the perimeter of any
rectangle. To use the formula, we replace each letter with a value. Have students use
the formula to find the perimeter of the 4-by-6 rectangle.
Use after Unit 4, Session 4.7
Now draw a 6-by-6 square on your grid paper. Find the perimeter. Discuss with
students their strategies. Let’s write a formula to find the perimeter of any square.
Remember that there are 4 sides all the same length. What letter should we use
to stand for the length of a side? Lead students to conclude that they can use the
following formula: P ⫽ 4 ⫻ s.
Sketch a variety of rectangles and squares on the board, as well as figures made up of
combinations of rectangles and squares. Label the side lengths and have students find
the perimeter of each figure. Have them explain their strategies.
© Pearson Education, Inc. 4
PR AC TICE
In the Activity 24 Master, students find the perimeter of rectangles, squares, and
combinations of rectangles and squares.
DIFFERENTIATION : Suppor ting the Range of Learner s
Have students explain how to find the length of a side of a square given
its perimeter. For example, they may divide the perimeter by 4. Then have them explain
how to find the width of a rectangle given its perimeter and its length. For example, they
may subtract two lengths from the perimeter and divide the difference by 2.
Unit 4: Size, Shape, and Symmetry
Session 4.7 (End-of-Unit Assessment)