The figure shows a large circle with radius m and four small circles with radii m. It is to be painted using the three shown colours. What is the cost of painting the figure?
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Problem 1419
Official solution
1. Calculate the area of the large circle:
The radius of the large circle is given as meters. The area of a circle is given by the formula:
Substituting the radius:
2. Calculate the area of one small circle:
The radius of each small circle is meter. Using the same formula for the area of a circle:
Since there are four small circles, the total area of the small circles is:
3. Calculate the area of the dark grey region:
The dark grey region is the area of the large circle minus the area of the four small circles plus the area of the light grey region (since the small circles overlap and we subtract too much).
Substituting the known areas:
Simplifying, we find:
4. Calculate the area of the light grey region:
To find the area of the light grey region, consider the overlapping area of two small circles. The overlapping region can be found by calculating the area of the sector minus the area of the triangle formed by the radii and the chord.
Therefore, the area of one overlapping region is:
Since there are four such overlapping regions:
5. Calculate the area of the medium grey region:
The medium grey region is the area of the four small circles minus the area of the light grey region:
Substituting the known areas:
6. Calculate the cost of painting the figure:
The cost of painting each region is given by:
- Dark grey region:
- Medium grey region:
- Light grey region:
Therefore, the total cost is:
Substituting the areas:
Simplifying:
The final answer is