2000 AMC 8 Problem 19

Attempt Problem 19 of the 2000 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2000 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

19.

Three circular arcs of radius 55 units bound the region shown. Arcs ABAB and ADAD are quarter-circles, and arc BCDBCD is a semicircle. What is the area, in square units, of the region?
2525
10+5π10+5\pi
5050
50+5π50+5\pi
25π25\pi
Answer: C
Concepts:area decompositioncircle area
Small hint:
Move the curved pieces to make a rectangle
Big hint:
The rearranged rectangle has dimensions 55 and 1010
Solution:
Create a rectangle that covers the bottom half of the figure as shown below.
Then, we get that [ABCD]=[ABD]+[BCD]. [ABCD] = [ABD] + [BCD].
We also know that [ABD]=[BDEF]−[ABE] [ABD] = [BDEF] - [ABE]−[ADF]. - [ADF].
ABEABE and ADFADF are both quarter-circles that form a semicircle with the same area as BCD.BCD.
This means that [ABD]=[BDEF]−[BCD] [ABD] = [BDEF] - [BCD] and [ABCD]=[BCD]+[BDEF] [ABCD] = [BCD] + [BDEF]−[BCD] - [BCD] =[BDEF]=10⋅5=50. = [BDEF] = 10 \cdot 5 = 50.
Thus, C is the correct answer.
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