2024 AMC 8 Problem 18

Below is the professionally curated solution for Problem 18 of the 2024 AMC 8, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2024 AMC 8 solutions, or check the answer key.

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Concepts:annulussectorlinear equation

Difficulty rating: 1540

18.

Three concentric circles centered at OO have radii of 1,1, 2,2, and 3.3. Points BB and CC lie on the largest circle. The region between the two smaller circles is shaded, as is the portion of the region between the two larger circles bounded by central angle BOC,BOC, as shown in the figure below. Suppose the shaded and unshaded regions are equal in area. What is the measure of BOC\angle BOC in degrees?

108108

120120

135135

144144

150150

Solution:

Let θ\theta be the measure of BOC.\angle BOC.

One component of the shaded region is the area of the circle with radius 22 minus the area of the circle with radius 1.1. This part has area 4ππ=3π.4\pi-\pi=3\pi. The remaining area is a sector of the biggest circle minus the area of the circle with radius 22. This has area θ360(9π4π)=θ360(5π).\dfrac{\theta}{360}(9\pi-4\pi) = \dfrac{\theta}{360}(5\pi). Hence, the total area of the shaded region is 3π+θ360(5π).3\pi + \dfrac{\theta}{360}(5\pi).

Next, we note that the unshaded region is composed of the smallest circle and the unshaded portion of the outer ring. This will have a total area of π+360θ360(5π)\pi + \dfrac{360-\theta}{360}(5\pi)

Lastly, we equate the area of both regions and solve for θ:\theta: 3π+θ360(5π)=π+360θ360(5π) 3\pi + \dfrac{\theta}{360}(5\pi) = \pi + \dfrac{360-\theta}{360}(5\pi) 2π=360θθ360(5π) 2\pi = \dfrac{360-\theta-\theta}{360}(5\pi) 25=12θ360 \dfrac{2}{5} = 1 - \dfrac{2\theta}{360} 2θ=35(360) 2\theta = \dfrac{3}{5}(360) θ=108. \theta = 108.

Thus, A is the correct answer.

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