Maths Olympiad Prep

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Geometry Difficulty 4.8 AIME Prove it Philippines

Problem:

Inside square ABCDA B C D, a point EE is chosen so that triangle DECD E C is equilateral. Find the measure of AEB\angle A E B.

Figure 1

Solution

Solution:

Since DEC\triangle D E C is an equilateral triangle, then DE=CE|D E| = |C E| and each angle has a measure of 6060^{\circ}. This implies that ADE\angle A D E has measure of 3030^{\circ}. Since AD=DE|A D| = |D E|, then ADE\triangle A D E is an isosceles triangle. Thus, DAE=DEA=75\angle D A E = \angle D E A = 75^{\circ}. The same argument on triangle BECB E C will give us CBE=CEB=75\angle C B E = \angle C E B = 75^{\circ}. Thus, AEB=150\angle A E B = 150^{\circ}.

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