Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Prove it United States

Problem:
Let ABCDABCD be a quadrilateral with an inscribed circle centered at II. Let CICI intersect ABAB at EE. If IDE=35\angle IDE = 35^{\circ}, ABC=70\angle ABC = 70^{\circ}, and BCD=60\angle BCD = 60^{\circ}, then what are all possible measures of CDA\angle CDA?

Solution

Solution:
Arbitrarily defining BB and CC determines II and EE up to reflections across BCBC. DD lies on both the circle determined by EDI=35\angle EDI = 35^{\circ} and the line through CC tangent to the circle (and on the opposite side of BB); since the intersection of a line and a circle has at most two points, there are only two cases for ABCDABCD. The diagram below on the left shows the construction made in this solution, containing both cases. The diagram below on the right shows only the degenerate case.

Figure 1

Reflect BB across ECEC to BB', then CB=CBCB = CB'. Since BABA and BCBC are tangent to the circle centered at II, IBIB is the angle bisector of ABC\angle ABC. Then IBE=IBE=35\angle IBE = \angle IB'E = 35^{\circ}. If B=DB' = D, then ADC=EBC=70\angle ADC = \angle EB'C = 70^{\circ}. Otherwise, since IBE=35=IDE\angle IB'E = 35^{\circ} = \angle IDE (given), EBDIEB'DI is a cyclic quadrilateral. Then IED=IBD=35\angle IED = \angle IB'D = 35^{\circ} and BCI=ECD=30\angle BCI = \angle ECD = 30^{\circ}, so CEDCBI\triangle CED \sim \triangle CBI.
Since CID\angle CID is exterior to DIE\triangle DIE, CID=IDE+DEI=70\angle CID = \angle IDE + \angle DEI = 70^{\circ}. Then CDICEB\triangle CDI \sim \triangle CEB. Because EBDIEB'DI is cyclic, IDC=IEB=IEB=1807030=80\angle IDC = \angle IEB' = \angle IEB = 180^{\circ} - 70^{\circ} - 30^{\circ} = 80^{\circ}. Then ADC=2IDC=160\angle ADC = 2 \angle IDC = 160^{\circ}.

Thus, the two possible measures are 7070^{\circ} and 160160^{\circ}.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.