Find the Perimeter
Aligned to 3.MD.D.8 — Common Core State Standards for Mathematics.
What this lesson teaches
Perimeter is the distance all the way around a shape. Add up the lengths of every side to find it.
Worked examples
Example 1
A rectangle has sides of 4, 4, 2, and 2. Add them all: 4 + 4 + 2 + 2 = 12. The perimeter is 12 units.
Example 2
A triangle has sides of 5, 6, and 7. Add them: 5 + 6 + 7 = 18. The perimeter is 18 units.
Example 3
Two shapes can share a perimeter but not an area. A 1-by-5 rectangle and a 2-by-4 rectangle both have a perimeter of 12, but their areas are 5 and 8.
Practice questions
- Find the perimeter of a square with sides of 5.
- A triangle has sides 3, 4, and 5. What is its perimeter?
- A rectangle has a perimeter of 10 and three sides of 3, 2, and 3. What is the missing side?
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Hallar el Perímetro
El perímetro es la distancia alrededor de toda una figura. Suma las longitudes de todos los lados para hallarlo.
Ejemplo 1: Un rectángulo tiene lados de 4, 4, 2 y 2. Súmalos todos: 4 + 4 + 2 + 2 = 12. El perímetro es 12 unidades.
Ejemplo 2: Un triángulo tiene lados de 5, 6 y 7. Súmalos: 5 + 6 + 7 = 18. El perímetro es 18 unidades.
Ejemplo 3: Dos figuras pueden tener el mismo perímetro pero no la misma área. Un rectángulo de 1 por 5 y uno de 2 por 4 tienen perímetro 12, pero sus áreas son 5 y 8.
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