1994 AMC 12 Problem 17

Attempt Problem 17 of the 1994 AMC 12 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 1994 AMC 12 solutions, or check the answer key.

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

17.

An 88 by 222\sqrt2 rectangle has the same center as a circle of radius 2.2. The area of the region common to both the rectangle and the circle is
2π2\pi
2π+22\pi+2
4π−44\pi-4
2π+42\pi+4
4π−24\pi-2
Answer: D
Concepts:circle segmentsareasymmetry
Small hint:
The rectangle removes two congruent caps from the circle
Big hint:
The cap chord is at distance 2\sqrt2 from the center, so its half central angle is 45∘45^\circ
Solution:
The rectangle is wider than the circle, so the common region is the circle with the top and bottom caps beyond distance 2\sqrt2 from the center removed. One cap has area 22(π4)−222−(2)2=π−2. \begin{aligned} 2^2\left(\frac{\pi}{4}\right) &-\sqrt2\sqrt{2^2-(\sqrt2)^2}\\ &=\pi-2. \end{aligned} Therefore the common area is 4π−2(π−2)=2π+4.4\pi-2(\pi-2)=2\pi+4. Thus the correct answer is D.
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