1995 AMC 12 Problem 17

Attempt Problem 17 of the 1995 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 1995 AMC 12 solutions, or check the answer key.

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

17.

Given regular pentagon ABCDE,ABCDE, a circle can be drawn that is tangent to DC‾\overline{DC} at DD and to AB‾\overline{AB} at A.A. The number of degrees in minor arc AD⌢\overset{\frown}{AD} is
7272
108108
120120
135135
144144
Answer: E
Concepts:regular polygonstangentscentral angles
Small hint:
The radii to AA and DD are perpendicular to the tangent sides
Big hint:
Use the 72∘72^\circ exterior angle of a regular pentagon to compare AB‾\overline{AB} and DC‾\overline{DC}
Solution:
Let OO be the circle’s center. The direction changes by 72∘72^\circ at each vertex of the pentagon, so the acute angle between lines ABAB and DCDC is 36∘.36^\circ. The radii to their tangency points are perpendicular to these lines; for the circle shown, the minor central angle ∠AOD\angle AOD is the supplementary angle 180∘−36∘=144∘.180^\circ-36^\circ=144^\circ. Therefore minor arc AD⌢\overset{\frown}{AD} measures 144∘,144^\circ, and the correct answer is E.
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