Thinkabara

🧮 3rd Grade · Math3.MD.D.8Unit 11: Perimeter & shapes

Unknown side lengths

Google Classroom

🎯 Objective: Students find a missing side when the perimeter is known.

Lesson outline

  1. A rectangle has perimeter 20 cm and one side 6 cm. Find the other sides.
  2. Model with a drawing and equation.
  3. Solve puzzles with missing sides.
  4. Exit: one missing side.

Lesson 2 of 5 in this unit. In the Lesson Planner this lesson takes 1–3 class periods depending on your timetable: day 1 introduce & model, then practice, small-group reteach and a quick check.

📐 Standard

3.MD.D.8

Common Core State Standards for Mathematics, Grade 3 — typical US scope & sequence

🏁 Unit goals

  • Find perimeter and unknown side lengths
  • Classify quadrilaterals by their attributes

✅ Check for understanding

End with the exit ticket in the outline. Sort answers into got it / almost / not yet and start the next lesson with a 5-minute reteach for the “not yets”.

Practice & resources on Thinkabara

🎯 Practice: Area & perimeter📚 All 3rd Grade skills🎮 Learning games⚡ Capy Live class quiz

🗓️ Plan the whole year like this

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More lessons in “Perimeter & shapes”

  1. What is perimeter?
  2. Unknown side lengths
  3. Same perimeter, different area
  4. Quadrilaterals
  5. Partition shapes into equal areas

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