CCSS.Math.Content.3.MD.D.8
The standard
Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.
Common Core State Standards for Mathematics · Measurement and Data
What this standard means
Students need to find the perimeter of polygons by adding side lengths, including shapes where not every side is labeled. They also need to use a known perimeter to find a missing side length. For rectangles, they should see that perimeter and area are different measurements.
Mastery looks like choosing the right operation, labeling units, and explaining why two rectangles can share a perimeter but have different areas. Students often mix up area and perimeter, forget to count all sides, or assume same area means same perimeter.
Ways to teach it
- Give pairs grid paper and tiles to build three rectangles with perimeter 20, then record each area.
- Ask students to explain: Can two dog pens use the same fencing but cover different amounts of grass?
- Show a polygon with one missing side and perimeter 36, then have students write and solve the equation.
- Measure a classroom object with string, then compare its perimeter to the area it covers on a desk or floor.
Plan a lesson for CCSS.Math.Content.3.MD.D.8
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