Maths Olympiad Prep

Track / Stage 7 / 19 of 300 #1419 of 1964

Problem 1419

National olympiad second round; IMO P1/P4
Geometry Difficulty 7.0 Find the answer

The figure shows a large circle with radius 22 m and four small circles with radii 11 m. It is to be painted using the three shown colours. What is the cost of painting the figure?
[img]https://1.bp.blogspot.com/-oWnh8uhyTIo/XzP30gZueKI/AAAAAAAAMUY/GlC3puNU_6g6YRf6hPpbQW8IE8IqMP3ugCLcBGAsYHQ/s0/2018%2BMohr%2Bp2.png[/img]

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solution

1. Calculate the area of the large circle:
The radius of the large circle is given as 22 meters. The area AA of a circle is given by the formula:
A=πr2 A = \pi r^2
Substituting the radius:
Alarge=π(2)2=4π square meters A_{\text{large}} = \pi (2)^2 = 4\pi \text{ square meters}

2. Calculate the area of one small circle:
The radius of each small circle is 11 meter. Using the same formula for the area of a circle:
Asmall=π(1)2=π square meters A_{\text{small}} = \pi (1)^2 = \pi \text{ square meters}
Since there are four small circles, the total area of the small circles is:
4Asmall=4π square meters 4A_{\text{small}} = 4\pi \text{ square meters}

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).
Adark=Alarge4Asmall+Alight A_{\text{dark}} = A_{\text{large}} - 4A_{\text{small}} + A_{\text{light}}
Substituting the known areas:
Adark=4π4π+Alight A_{\text{dark}} = 4\pi - 4\pi + A_{\text{light}}
Simplifying, we find:
Adark=Alight A_{\text{dark}} = A_{\text{light}}

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.
Area of sector=90360π(1)2=π4 \text{Area of sector} = \frac{90^\circ}{360^\circ} \pi (1)^2 = \frac{\pi}{4}
Area of triangle=1×12=12 \text{Area of triangle} = \frac{1 \times 1}{2} = \frac{1}{2}
Therefore, the area of one overlapping region is:
Area of one overlapping region=π412 \text{Area of one overlapping region} = \frac{\pi}{4} - \frac{1}{2}
Since there are four such overlapping regions:
Alight=4(π412)=π2 A_{\text{light}} = 4 \left( \frac{\pi}{4} - \frac{1}{2} \right) = \pi - 2

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:
Amedium=4AsmallAlight A_{\text{medium}} = 4A_{\text{small}} - A_{\text{light}}
Substituting the known areas:
Amedium=4π(π2)=4ππ+2=3π+2 A_{\text{medium}} = 4\pi - (\pi - 2) = 4\pi - \pi + 2 = 3\pi + 2

6. Calculate the cost of painting the figure:
The cost of painting each region is given by:
- Dark grey region: 30 kr. per square meter30 \text{ kr. per square meter}
- Medium grey region: 20 kr. per square meter20 \text{ kr. per square meter}
- Light grey region: 10 kr. per square meter10 \text{ kr. per square meter}

Therefore, the total cost is:
Total cost=30Adark+20Amedium+10Alight \text{Total cost} = 30A_{\text{dark}} + 20A_{\text{medium}} + 10A_{\text{light}}
Substituting the areas:
Total cost=30(π2)+20(3π+2)+10(π2) \text{Total cost} = 30(\pi - 2) + 20(3\pi + 2) + 10(\pi - 2)
Simplifying:
Total cost=30π60+60π+40+10π20 \text{Total cost} = 30\pi - 60 + 60\pi + 40 + 10\pi - 20
Total cost=100π40 \text{Total cost} = 100\pi - 40

The final answer is 100π40 kr.\boxed{100\pi - 40 \text{ kr.}}

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.