Cogito
Grade 7 Math · Chapter 7 · Lesson 2
Area of a Circle
A different question, with different units.
10 problems · about 18 minutes · 7.G.B.4
What this lesson teaches
The child computes the area of a circle and tells area apart from circumference.
- Area of a circle is πr².
- Circumference uses the diameter; area uses the radius squared.
- The units tell the two apart.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Measuring a Circle: Diameter 50, using π = 3.14. Circumference?
Answer 157
Why 157.
Review — Solving Inequalities: 4x > 20. Smallest whole number x?
Answer 6
Why 6.
Warm Up
Straightforward practice. Get the method working first.
4 problemsRadius 4, π = 3.14. Area?
Answer 50.24
Why 50.24 cm².
How do you tell area from circumference in a question?
Answer By the units: cm² is area, cm is length.
Why The units say which one is wanted.
Radius 2, π = 3.14. Area?
Answer 12.56
Why 3.14 × 4.
Radius 6, π = 3.14. Area?
Answer 113.04
Why 3.14 × 36.
Build It Up
The same ideas with more to keep track of.
2 problemsRadius 5, π = 3.14. Circumference?
Answer 31.4
Why Diameter is 10.
Which formula uses the radius squared?
Answer Area.
Why A = πr².
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Table: A circle has radius 10 cm. What is its area, using π = 3.14?
Answer 314 square cm
Why 314 cm².
The Comparison: A circle has radius 3 cm. What is its area, using π = 3.14?
Answer 28.26 square cm
Why 28.26 cm².