2014 AMC 8 Problem 20

Attempt Problem 20 of the 2014 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 2014 AMC 8 solutions, or check the answer key.

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

20.

Rectangle ABCDABCD has sides CD=3CD=3 and DA=5.DA=5. A circle of radius 11 is centered at A,A, a circle of radius 22 is centered at B,B, and a circle of radius 33 is centered at C.C. Which of the following is closest to the area of the region inside the rectangle but outside all three circles?
3.5 3.5
4.0 4.0
4.5 4.5
5.0 5.0
5.5 5.5
Answer: B
Concepts:sectorareaestimation
Small hint:
Inside the rectangle, each circle contributes a quarter-circle sector.
Big hint:
Subtract the total sector area from the rectangle area and approximate π\pi.
Solution:
The parts of the three circles inside the rectangle are quarter-circles with radii 11, 22, and 33. Their total area is π4+π+9π4=7π2.\dfrac{\pi}{4}+\pi+\dfrac{9\pi}{4}=\dfrac{7\pi}{2}.
Using π≈227\pi\approx\dfrac{22}{7}, this total is about 1111. The rectangle has area 3⋅5=153\cdot5=15, so the desired area is about 15−11=415-11=4.
Thus, B is the correct answer.
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