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Grade 7 · Math · Unit 16 of 20

Circles

Circumference, area and π

What your child practises

Using radius, diameter and π (about 3.14) to find the distance around a circle and the area inside it, including real objects like wheels and pizzas.

Ontario Curriculum expectation: E2.3–E2.5 · circumference and area of circles

How we explain it

Children can open this short lesson before they practise.

  1. The radius goes from the centre to the edge. The diameter goes all the way across: d = 2r.
  2. Circumference (the distance around) is C = π × d.
  3. Area is A = π × r × r. Use π ≈ 3.14.

Example: A circle has diameter 10 cm. What is its circumference? C = π × d. C ≈ 3.14 × 10 = 31.4. Answer: About 31.4 cm

Sample questions

A taste of this unit. In the app, questions change every time and get easier or harder to match your child.

  1. Question 1

    A circle has a diameter of 32 cm. What is its circumference? Use 3.14 for π and round to one decimal place.

    ⭕d = 32 cm

    answer boxcm

    Show answer

    100.5 cm

    Hint: C = π × d ≈ 3.14 × 32 = 100.48. Rounded to one decimal place: 100.5 cm.

  2. Question 2

    A circle has a radius of 11 cm. What is its area? Use 3.14 for π and round to one decimal place.

    ⭕r = 11 cm

    answer boxcm²

    Show answer

    379.9 cm²

    Hint: A = π × r × r ≈ 3.14 × 11 × 11 = 3.14 × 121 = 379.94. Rounded: 379.9 cm².

  3. Question 3

    A circle has a radius of 24 cm. What is its diameter?

    ⭕r = 24 cm
    • 12 cm
    • 96 cm
    • 48 cm
    • 24 cm
    Show answer

    48 cm

    Hint: The diameter is two radii laid end to end: 2 × 24 = 48 cm.

  4. Question 4

    A circle has a radius of 23 m. What is its circumference? Use 3.14 for π and round to one decimal place.

    ⭕r = 23 m

    answer boxm

    Show answer

    144.4 m

    Hint: C = 2 × π × r ≈ 2 × 3.14 × 23 = 144.44. Rounded: 144.4 m. (Or double the radius to get d = 46, then 3.14 × d.)

  5. Question 5

    A circle's circumference is 37.68 cm. What is its diameter? (Use 3.14 for π.)

    answer boxcm

    Show answer

    12 cm

    Hint: C = 3.14 × d, so d = C ÷ 3.14 = 37.68 ÷ 3.14 = 12 cm.

  6. Question 6

    A circle has a diameter of 28 cm. Using 3.14 for π, what is its area?

    ⭕d = 28 cm
    • 87.92 cm²
    • 2461.76 cm²
    • 615.44 cm²
    • 43.96 cm²
    Show answer

    615.44 cm²

    Hint: First find the radius: 28 ÷ 2 = 14. Then A = 3.14 × 14 × 14 = 615.44 cm². (Using the diameter instead of the radius makes the area 4 times too big.)

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