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Geometry Difficulty 4.4 AIME Prove it Hong Kong

Let ABCDABCD be a trapezoid with ABCDAB \parallel CD, AB>CDAB > CD and AD>BCAD > BC. Show that

i. CBA>DAB\angle CBA > \angle DAB,

ii. AC>BDAC > BD.

Solution

i. Let ADAD meet BCBC at EE, ACAC meet BDBD at FF, and let MM be the midpoint of ABAB. Since AB>CDAB > CD, the points DD and CC must lie on the segments AEAE and BEBE respectively.
Since EDCEAB\triangle EDC \sim \triangle EAB, we have EAEB=DACB>1\frac{EA}{EB} = \frac{DA}{CB} > 1. This implies EA>EBEA > EB, and hence CBA>DAB\angle CBA > \angle DAB.

Figure 1

ii. By Steiner's theorem, EE, FF, MM are collinear. Since EA>EBEA > EB, both EE and FF lie on the same side as BB to the perpendicular bisector of ABAB. This implies FA>FBFA > FB. Now, as FABFCD\triangle FAB \sim \triangle FCD, we have ACBD=FAFB>1\frac{AC}{BD} = \frac{FA}{FB} > 1. This means AC>BDAC > BD.

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