Maths Olympiad Prep

Library / Test 1 /2 of 6

Test 1

Geometry Difficulty 4.4 AIME Prove it Hong Kong

Let ABCDABCD be a trapezoid with AB∥CDAB \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 △EDC∼△EAB\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 △FAB∼△FCD\triangle FAB \sim \triangle FCD, we have ACBD=FAFB>1\frac{AC}{BD} = \frac{FA}{FB} > 1. This means AC>BDAC > BD.

Want a route through all this instead of an archive? The track puts 2,604 problems in a working order, from Junior Challenge level to the IMO shortlist.

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