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Perimeter and Circumference
Students measure distance around polygons and circles and solve for missing dimensions.
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At a Glance
How do dimensions determine a figure’s perimeter and area?
Perimeter is the total distance around a polygon. Add every side length, using equal-side markings and algebra when a length is unknown.
Circumference is the distance around a circle. Use C = pi times d or C = 2 times pi times r. The diameter is twice the radius. Keep pi in exact answers or use the requested approximation.
4,327 = 4,000 + 300 + 20 + 7
Learning Targets
- Add all outside side lengths.
- Use P = 2l + 2w for rectangles.
- Use C = pi d or C = 2 pi r.
- Label answers with linear units.
- Work backward to find missing dimensions.
Interactive Vocabulary
Add a student-friendly definition in the Binder Page editor.
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Worked Examples
Rectangle 8 cm by 5 cm
P=2(8+5)=26 cm.
A=8×5=40 cm².
Triangle with base 10 m and height 7 m
A=1/2(10)(7)=35 m².
Common Questions
Is perimeter measured in square units? No, it uses linear units.
Can I use 3.14 for pi? Only when directions request an approximation.
Are radius and diameter interchangeable? No; d = 2r.
Common Misconceptions
Using square units for perimeter
Perimeter uses linear units.
Adding dimensions for area
Area measures covering and uses multiplication or decomposition.
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Practice until you can explain it
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Warm-Up
Find the perimeter and area of a 9-by-4 rectangle.
Guided Practice
Rectangle 8 by 5: P = 26 units.
Triangle sides 6, 9, and 11: P = 26 units.
Circle radius 7: C = 14 pi units.
Circle diameter 12: C = 12 pi units.
A square has perimeter 36: side = 9 units.
Independent Practice
Find the perimeter and area of a 9-by-4 rectangle.
Create and solve a second example with different values.
Challenge & Real-World Practice
Design two rectangles with area 72 square units but different perimeters. Explain why equal area does not require equal perimeter.
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