2019 AMC 10B Problems/Problem 20
- The following problem is from both the 2019 AMC 10B #20 and 2019 AMC 12B #15, so both problems redirect to this page.
Contents
Problem
As shown in the figure, line segment is trisected by points and so that Three semicircles of radius and have their diameters on and are tangent to line at and respectively. A circle of radius has its center on The area of the region inside the circle but outside the three semicircles, shaded in the figure, can be expressed in the form where and are positive integers and and are relatively prime. What is ?
Solution
Divide the circle into four parts: The top semicircle (1), the bottom sector with arc length 120 degrees (2), the triangle formed by the radii of (2) and the chord (3), and the four parts which are the corners of a circle inscribed in a square. The area is just (1) + (2) - (3) + (4).
Area of (1):
Area of (2):
Area of (3): Radius of 2, distance of 1 to BC, creates 2 30-60-90 triangles, so area of it is
Area of (4):
Total sum:
-Arpitr20
Solution 2
I don't have enough time to post it yet, but here is the outline: Divide it into the top semicircle, the bottom sector, and two rectangles. Using these rectangles we can find the area of those weird shapes that are shaded. Adding everything together, and we can finish. Sorry for not the full solution.
See Also
2019 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 19 |
Followed by Problem 21 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
2019 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 14 |
Followed by Problem 16 |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | |
All AMC 12 Problems and Solutions |
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