Cogito
Grade 3 Math · Chapter 4 · Lesson 5
Perimeter
The distance all the way round.
10 problems · about 12 minutes · 3.MD.D.8
What this lesson teaches
The child finds the perimeter of a polygon by adding its side lengths.
- Perimeter is the distance all the way round.
- Add every side of the shape.
- It is a length, so no square units.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Breaking a Big Area Apart: Area of 5 by 9, split as 5 by 5 and 5 by 4?
Answer 45
Why 25 + 20 = 45 square units.
Review — Finding a Missing Side: Area 42, one side 6. What is the other side?
Answer 7
Why The other side is 7.
Review — Area by Multiplying: A rectangle 8 by 7. What is its area?
Answer 56
Why The area is 56 square units.
Warm Up
Straightforward practice. Get the method working first.
3 problemsA rectangle 9 by 5. What is its perimeter?
Answer 28
Why The perimeter is 28 units.
What units does a perimeter use?
Answer Units, because it is a length.
Why A perimeter is a distance, so it uses plain length units.
A rectangle 7 by 3. What is its perimeter?
Answer 20
Why 7 + 3 + 7 + 3.
Build It Up
The same ideas with more to keep track of.
1 problemA square with sides of 6. What is its perimeter?
Answer 24
Why Four sides of 6.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problemsA rectangle 10 by 2. What is its perimeter?
Answer 24
Why 10 + 2 + 10 + 2.
The Fence: A garden is 8 m by 6 m. How much fence goes all the way round?
Answer 28 metres
Why The perimeter is 28 metres.
The Picture Frame: A frame is 9 cm by 4 cm. What is its perimeter?
Answer 26 cm
Why The perimeter is 26 cm.