2021 AMC 10A Fall Problem 7

Below is the professionally curated solution for Problem 7 of the 2021 AMC 10A Fall, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2021 AMC 10A Fall solutions, or check the answer key.

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Concepts:angle chasingisosceles trianglesquare (geometry)

Difficulty rating: 960

7.

As shown in the figure below, point EE lies on the opposite half-plane determined by line CDCD from point AA so that CDE=110.\angle CDE = 110^\circ. Point FF lies on AD\overline{AD} so that DE=DF,DE=DF, and ABCDABCD is a square. What is the degree measure of AFE?\angle AFE?

160160

164164

166166

170170

174174

Solution:

Since ADC=90,\angle ADC = 90^{\circ}, we get that FDE=36090110 \angle FDE = 360^{\circ} - 90^{\circ} - 110^{\circ} =160.= 160^{\circ}. Also since FDE\triangle FDE is isosceles, we get that EFD=1801602=10. \angle EFD = \dfrac{180^{\circ} - 160^{\circ}}{2} = 10^{\circ}. Finally, we get that AFE=18010=170. \angle AFE = 180^{\circ} - 10^{\circ} = 170^{\circ}.

Thus, D is the correct answer.

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