Cogito
Grade 3 Math · Chapter 4 · Lesson 6
Telling Area and Perimeter Apart
Two different questions about the same shape.
10 problems · about 14 minutes · 3.MD.C.7, 3.MD.D.8
What this lesson teaches
The child distinguishes area from perimeter and chooses the right one for a situation.
- Area is the space inside. Perimeter is the distance around.
- Area uses square units. Perimeter uses plain units.
- Two rectangles can share a perimeter and have different areas.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
1 problemReview — Perimeter: A rectangle 9 by 5. What is its perimeter?
Answer 28
Why The perimeter is 28 units.
Warm Up
Straightforward practice. Get the method working first.
4 problemsA rectangle 9 by 4. What is its area?
Answer 36
Why The area is 36 square units.
A rectangle 9 by 4. What is its perimeter?
Answer 26
Why The perimeter is 26 units.
Paint for a wall. Area or perimeter?
Answer Area
Why Paint covers the inside.
Edging round a flower bed. Area or perimeter?
Answer Perimeter
Why Edging goes around.
Build It Up
The same ideas with more to keep track of.
2 problemsA rectangle 8 by 3. What is its area?
Answer 24
Why 8 × 3.
A rectangle 8 by 3. What is its perimeter?
Answer 22
Why 8 + 3 + 8 + 3.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problemsThe Carpet: A room is 7 m by 4 m. How much carpet covers the floor?
Answer 28 square metres
Why 7 × 4 = 28 square metres.
The Ribbon: A box top is 7 cm by 4 cm. How much ribbon goes round the edge?
Answer 22 cm
Why 7 + 4 + 7 + 4 = 22 cm.
Same Perimeter: A 6 by 2 rectangle and a 5 by 3 rectangle both have perimeter 16. What is the area of the 5 by 3?
Answer 15 square units
Why Same perimeter, but areas of 12 and 15. They are different quantities.