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2022 AMC 10B Problem 20

Problem 20 of 25HarderGeometry

Let ABCDABCD be a rhombus with ∠ADC=46∘.\angle ADC = 46^\circ. Let EE be the midpoint of CD‾,\overline{CD}, and let FF be the point on BE‾\overline{BE} such that AF‾\overline{AF} is perpendicular to BE‾.\overline{BE}. What is the degree measure of ∠BFC?\angle BFC?

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Solution

Extend BE‾\overline{BE} to meet line ADAD at G.G. Because AD∥BC,AD\parallel BC, we have ∠GDE=∠ECB,\angle GDE=\angle ECB, and ∠GED=∠BEC\angle GED=\angle BEC are vertical angles. Also DE=EC,DE=EC, so △GDE≅△BCE.\triangle GDE\cong\triangle BCE. Hence DG=BC=AD.DG=BC=AD. Thus DD is the midpoint of AG‾.\overline{AG}. The circle centered at DD through AA also passes through CC and G.G. Since AF⊥FG,AF\perp FG, Thales’ theorem places FF on this circle as well. Because DGDG is opposite DA,DA, ∠GDC=180∘−∠ADC=134∘.\begin{aligned}\angle GDC&=180^\circ-\angle ADC\\&=134^\circ.\end{aligned} The inscribed angle ∠GFC\angle GFC subtending arc GCGC is therefore 67∘.67^\circ. Finally, B,F,GB,F,G are collinear, so ∠BFC=180∘−67∘=113∘.\angle BFC=180^\circ-67^\circ=113^\circ. Thus, the answer is D .
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Tagged: angle chasing · cyclic quadrilateral · inscribed angle

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