2017 AMC 12A Problem 11

Attempt Problem 11 of the 2017 AMC 12A 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 2017 AMC 12A solutions, or check the answer key.

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11.

Claire adds the degree measures of the interior angles of a convex polygon and arrives at a sum of $2017.$ She then discovers that she forgot to include one angle. What is the degree measure of the forgotten angle?

$37$

$63$

$117$

$143$

$163$

Answer: D

Concepts:angle sum bounding to limit cases

Difficulty rating: 1570

Solution:

If the polygon has $n$ sides and the forgotten angle is $\alpha,$ then $(n-2)180=2017+\alpha.$ Since $0\lt\alpha\lt180,$ $2017\lt(n-2)180\lt2197.$

The only multiple of $180$ in this range is $2160=(14-2)180,$ so $n=14$ and $\alpha=2160-2017=143.$

Thus, the correct answer is D.

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