Visual Model 1
Question: What is the perimeter of this rectangle in units?
- A. \(9\) units
- B. \(18\) units
- C. \(36\) units
- D. \(54\) units
Why it works: \(P = 2(6) + 2(3) = 12 + 6 = 18\) units.
Answer: \(18\) units
CH06-10 lesson
Perimeter of Polygons Learning Activity
A focused Grade 3 learning session for Perimeter of Polygons using visual models, worked examples, strategy guidance, and a short mastery check.
- 15
- minutes
- 6
- lesson parts
- 11
- practice items
Students will solve perimeter problems for polygons, including problems with an unknown side length.
Student goal: I can find the perimeter of a polygon and solve for a missing side.
Helpful first:
Adding and Subtracting Within 1000
Learn the Idea
Perimeter of Polygons means finding the distance around a polygon by adding the side lengths carefully.
- Read the question carefully and identify what is being asked.
- Choose a model, equation, table, or diagram that matches the situation.
- Solve one step at a time and keep units or labels attached.
- Use the answer explanation to check that the result makes sense.
See the Math
Follow a Worked Example
Example 1
Question: What is the perimeter of this rectangle in centimeters?
- A. \(7\) cm
- B. \(14\) cm
- C. \(10\) cm
- D. \(28\) cm
- \(P = 2(5) + 2(2) = 10 + 4 = 14\) cm.
Answer: \(14\) cm
Example 2
Question: What is the perimeter?
- A. \(9\) units
- B. \(18\) units
- C. \(28\) units
- D. \(35\) units
- \(P = 2(7) + 2(2) = 14 + 4 = 18\) units.
Answer: \(18\) units
Connect It to a Real Situation
Problem 1
Question: A rectangle has length \(10\) inches and width \(4\) inches. Find the perimeter.
- A. \(28\) in
- B. \(14\) in
- C. \(40\) in
- D. \(56\) in
Why it works: \(P = 2(10) + 2(4) = 20 + 8 = 28\) in.
Answer: \(28\) in
Avoid Mistakes and Choose a Strategy
- Rushing before identifying what the numbers represent.
- Choosing an operation that does not match the situation.
- Dropping labels, units, or context from the answer.
- Skipping the estimate or reasonableness check.
- Underline the question being asked.
- Use a model before jumping to computation.
- Write an equation that matches the story or picture.
- Explain the final answer in a sentence.
Remember the Key Idea
Perimeter of Polygons becomes easier when students connect the question to a model, use clear steps, and explain why the answer fits.
GOLDEN RULE
Understand the model before choosing the operation.
Solve before opening the answer
Use scratch paper. The explanation matters more than simply seeing whether the choice is correct.
warmup
2 questionsA rectangle has length \(8\) cm and width \(5\) cm. What is its perimeter?
- A \(13\) cm
- B \(26\) cm
- C \(40\) cm
- D \(80\) cm
Check answer and explanation
Answer B: \(26\) cm
Perimeter \(=2\ell+2w=2(8)+2(5)=16+10=26\) cm.
What is the perimeter of this rectangle in units?
- A \(9\) units
- B \(18\) units
- C \(36\) units
- D \(54\) units
Check answer and explanation
Answer B: \(18\) units
\(P = 2(6) + 2(3) = 12 + 6 = 18\) units.
guided
1 questionThis is a square with side length \(4\) m. What is its perimeter?
- A \(8\) m
- B \(12\) m
- C \(16\) m
- D \(24\) m
Check answer and explanation
Answer C: \(16\) m
A square has 4 equal sides. \(P = 4 + 4 + 4 + 4 = 16\) m.
practice
5 questionsA rectangle has length \(10\) inches and width \(4\) inches. Find the perimeter.
- A \(28\) in
- B \(14\) in
- C \(40\) in
- D \(56\) in
Check answer and explanation
Answer A: \(28\) in
\(P = 2(10) + 2(4) = 20 + 8 = 28\) in.
What is the perimeter of this rectangle in centimeters?
- A \(7\) cm
- B \(14\) cm
- C \(10\) cm
- D \(28\) cm
Check answer and explanation
Answer B: \(14\) cm
\(P = 2(5) + 2(2) = 10 + 4 = 14\) cm.
A square has a perimeter of \(20\) feet. What is the length of one side?
- A \(5\) ft
- B \(4\) ft
- C \(10\) ft
- D \(20\) ft
Check answer and explanation
Answer A: \(5\) ft
A square has 4 equal sides. \(20 \div 4 = 5\) ft per side.
What is the perimeter?
- A \(9\) units
- B \(18\) units
- C \(28\) units
- D \(35\) units
Check answer and explanation
Answer B: \(18\) units
\(P = 2(7) + 2(2) = 14 + 4 = 18\) units.
A rectangle's length is \(12\) cm. Its width is \(3\) cm. What is the perimeter?
- A \(15\) cm
- B \(36\) cm
- C \(30\) cm
- D \(72\) cm
Check answer and explanation
Answer C: \(30\) cm
\(P = 2(12) + 2(3) = 24 + 6 = 30\) cm.
exit
3 questionsThis is a square with side \(3\) inches. Find its perimeter.
- A \(9\) in
- B \(15\) in
- C \(12\) in
- D \(18\) in
Check answer and explanation
Answer C: \(12\) in
\(P = 4 \times 3 = 12\) in.
Which of these has a perimeter of \(24\) feet?
- A Square with side \(4\) ft
- B Rectangle with length \(6\) ft and width \(3\) ft
- C Square with side \(5\) ft
- D Rectangle with length \(8\) ft and width \(4\) ft
Check answer and explanation
Answer D: Rectangle with length \(8\) ft and width \(4\) ft
Option D is correct: \(2(8) + 2(4) = 24\) ft. Option A is \(16\) ft, option B is \(18\) ft, and option C is \(20\) ft.
A rectangle has perimeter \(20\) cm. If the length is \(6\) cm, what is the width?
- A \(4\) cm
- B \(2\) cm
- C \(8\) cm
- D \(14\) cm
Check answer and explanation
Answer A: \(4\) cm
\(P = 2\ell + 2w\), so \(20 = 2(6) + 2w = 12 + 2w\). Thus \(2w = 8\) and \(w = 4\) cm.
