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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 ADBC,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 GDEBCE.\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 AFFG,AF\perp FG, Thales’ theorem places FF on this circle as well. Because DGDG is opposite DA,DA, GDC=180ADC=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=18067=113.\angle BFC=180^\circ-67^\circ=113^\circ. Thus, the answer is D .

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Concepts: angle chasing · cyclic quadrilateral · inscribed angle

Problem text and solution from the LIVE past-contest archive. See also the AoPS wiki page for community solutions.