2010 AMC 8 Problem 19

Attempt Problem 19 of the 2010 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 2010 AMC 8 solutions, or check the answer key.

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

19.

The two circles pictured have the same center C.C. Chord AD‾\overline{AD} is tangent to the inner circle at B,B, ACAC is 10,10, and chord AD‾\overline{AD} has length 16.16. What is the area between the two circles?
 36π \ 36 \pi
 49π \ 49 \pi
 64π \ 64 \pi
 81π \ 81 \pi
 100π \ 100 \pi
Answer: C
Concepts:circle areachordPythagorean Theorem
Small hint:
Half the chord, the inner radius, and the outer radius form a right triangle.
Big hint:
The radius to a tangent point is perpendicular to the chord.
Solution:
The area between the two circles is the area of the larger circle minus the area of the smaller circle. This would be (AC)2π−(CB)2π(AC)^2\pi - (CB)^2 \pi =π(AC2−CB2).= \pi(AC^2 - CB^2). By the Pythagorean Theorem, we can get AC2−CB2=AB2.AC^2 - CB^2 = AB^2. Therefore, we need to find AB2π.AB^2 \pi.
Since ABAB is half of AD,AD, we get AB=8.AB = 8. This makes AB2π=64π.AB^2 \pi = 64 \pi.
Thus, the answer is C .
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