Math Problem Statement

Ruby has a circular piece of paper with a radius of 15 cm. She cuts a circular hole with a radius of 4 cm in the center of her piece of paper to make the shape shown below. Calculate the area of this shape. Give your answer in cm² to 1 d.p.

Solution

To calculate the area of the shape, we need to find the area of the larger circle and subtract the area of the smaller circle (which represents the hole cut in the paper).

  1. Area of the larger circle:
    The formula for the area of a circle is A=πr2A = \pi r^2, where rr is the radius.

    • For the larger circle, the radius is 15cm15 \, \text{cm}: Alarge=π(15)2=225πcm2A_{\text{large}} = \pi (15)^2 = 225\pi \, \text{cm}^2
  2. Area of the smaller circle (the hole):

    • For the smaller circle, the radius is 4cm4 \, \text{cm}: Asmall=π(4)2=16πcm2A_{\text{small}} = \pi (4)^2 = 16\pi \, \text{cm}^2
  3. Subtract the area of the smaller circle from the area of the larger circle: Ashape=AlargeAsmall=225π16π=209πcm2A_{\text{shape}} = A_{\text{large}} - A_{\text{small}} = 225\pi - 16\pi = 209\pi \, \text{cm}^2

  4. Calculate the numerical value: Using π3.1416\pi \approx 3.1416: Ashape209×3.1416=656.6cm2A_{\text{shape}} \approx 209 \times 3.1416 = 656.6 \, \text{cm}^2

Therefore, the area of the shape is approximately 656.6 cm² (to 1 decimal place).

Would you like further details or have any questions?

Here are 5 related questions to explore:

  1. What would the area be if the smaller circle had a radius of 6 cm?
  2. How does the shape change if the hole is not centered?
  3. How can we calculate the circumference of the outer circle?
  4. What is the ratio of the area of the hole to the total area of the paper?
  5. How would the area change if we scaled the radius of both circles by 50%?

Tip: The difference of areas between two concentric circles forms an annular ring, and you can generalize the formula as π(R2r2)\pi (R^2 - r^2) for quick calculations.

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Math Problem Analysis

Mathematical Concepts

Geometry
Area Calculation
Circles
Subtraction of Areas

Formulas

Area of a circle: A = πr^2
Difference of areas for concentric circles: A = π(R^2 - r^2)

Theorems

Area of a Circle

Suitable Grade Level

Grades 6-8