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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Difficulty rating: 1540
18.
Three concentric circles centered at have radii of and Points and 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 as shown in the figure below. Suppose the shaded and unshaded regions are equal in area. What is the measure of in degrees?
Solution:
Let be the measure of
One component of the shaded region is the area of the circle with radius minus the area of the circle with radius This part has area The remaining area is a sector of the biggest circle minus the area of the circle with radius . This has area Hence, the total area of the shaded region is
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
Lastly, we equate the area of both regions and solve for
Thus, A is the correct answer.
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