Cogito
Grade 7 Math · Chapter 7 · Lesson 4
Telling Area from Circumference
Which question is being asked.
10 problems · about 18 minutes · 7.G.B.4
What this lesson teaches
The child chooses between circumference and area according to what a problem needs.
- Circumference measures around the circle; area measures inside it.
- Circumference is 2πr, area is πr².
- The units asked for tell you which one is wanted.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Area of a Circle: Diameter 16. What is r² ?
Answer 64
Why 64.
Review — Area of a Circle: Radius 4, π = 3.14. Area?
Answer 50.24
Why 50.24 cm².
Review — Measuring a Circle: Diameter 50, using π = 3.14. Circumference?
Answer 157
Why 157.
Warm Up
Straightforward practice. Get the method working first.
3 problemsEdging a round rug. 1 for circumference, 2 for area.
Answer 1
Why Circumference.
Ribbon around a cake. 1 for circumference, 2 for area.
Answer 1
Why Around the edge.
Icing on top of a cake. 1 for circumference, 2 for area.
Answer 2
Why Covers the surface.
Build It Up
The same ideas with more to keep track of.
2 problemsAn answer in square metres. 1 for circumference, 2 for area.
Answer 2
Why Square units.
Radius 5. What is 2r?
Answer 10
Why Twice the radius.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Fence: Fencing a circular garden. Which is needed? Answer 1 for circumference, 2 for area.
Answer 1
Why Circumference.
The Turf: Laying turf on that garden. Which is needed? Answer 1 for circumference, 2 for area.
Answer 2
Why Area.