Unit 5
A C T I V I T Y 18
AC TIVIT Y
Using Perimeter Formulas
25 MIN
Distribute the Power Polygons™ triangles and regular polygons. Hold up any polygon
with all sides the same length. This is called a regular polygon. Why do you
suppose it is called a regular polygon? On the board, list the regular polygons that
students have shown, along with the number of sides.
Use after
Unit 5, Session 2.6
Vocabulary/Vocabulario
regular polygon/
polígono regular
perimeter/
perímetro
formula/
fórmula
Distribute millimeter rulers. Work together
to measure and record the side lengths of
a regular pentagon. The distance around
a figure is called its perimeter. Find the
perimeter of the regular pentagon.
Can you find a shortcut to finding the
perimeter of a regular pentagon? Explain.
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Students might say:
Materials/Materiales
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Discuss shortcuts for other regular polygons and for isosceles triangles. Suppose we
use s to stand for the length of a side of an equilateral triangle. Describe a rule
we could use for finding the perimeter. Explain that we often use P to represent
perimeter, and we can write P ⫽ 3 ⫻ s. We call P ⴝ 3 ⴛ s a formula. A formula is
an equation that shows a rule for a relationship among quantities. Discuss formulas
that could be used to find the perimeter of other polygons.
Activity 18
MASTER
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Measuring Polygons
mm
“We only needed to measure one side of a regular pentagon. We then
multiplied that length by 5 to find the perimeter. Since 5 ⴛ 20 ⴝ 100,
the perimeter is 100 millimeters.”
• Power Polygons™/
polígonos Power
• rulers marked in millimeters/
reglas marcadas en milímetros
• Activity 18 Master (see below )/
Actividad 18 (ver abajo)
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Choose two identical polygons and align them along one edge. Then find the
perimeter of the new, double shape. Is the new shape’s perimeter twice the
perimeter of one of the polygons? Explain.
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Use after Unit 5, Session 2.6
“No. I put two equilateral triangles together. The total perimeter of
one triangle is 3 ⴛ 14, or 42 millimeters. The two triangles pushed
together have 4 sides so the perimeter is 4 ⴛ 14, or 56 millimeters.
This is less than twice 42.”
Discuss how the perimeter changes when other shapes are combined.
PR AC TICE
In the Activity 18 Master, students find the perimeter of polygons.
DIFFERENTIATION : Suppor ting the Range of Learner s
Discuss how to solve problems such as these: How can you find the
length of one side of a regular hexagon if the perimeter is 54 inches? Suppose the
shortest side of an isosceles triangle is 7 inches long and the perimeter is 25 inches.
How can you find the length of the other sides?
Unit 5: Measuring Polygons
Session 2.6 (Assessment: Perimeter and Area of Rectangles)