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Junior Balkan MO

Geometry Difficulty 5.5 AIME, harder Find the answer

The triangle ABCABC is isosceles with AB=ACAB=AC, and ∠BAC<60∘\angle{BAC}<60^{\circ}. The points DD and EE are chosen on the side ACAC such that, EB=EDEB=ED, and ∠ABD≡∠CBE\angle{ABD}\equiv\angle{CBE}. Denote by OO the intersection point between the internal bisectors of the angles ∠BDC\angle{BDC} and ∠ACB\angle{ACB}. Compute ∠COD\angle{COD}.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Given triangle △ABC \triangle ABC , where AB=AC AB = AC and ∠BAC<60∘ \angle BAC < 60^\circ , points D D and E E are chosen on side AC AC such that EB=ED EB = ED and ∠ABD≡∠CBE \angle ABD \equiv \angle CBE . We are tasked with finding ∠COD\angle COD, where O O is the intersection of the internal bisectors of ∠BDC\angle BDC and ∠ACB\angle ACB.

### Step-by-Step Solution:

1. Understanding the Isosceles Triangle:
Since AB=AC AB = AC , triangle △ABC \triangle ABC is isosceles. Given that ∠BAC<60∘\angle BAC < 60^\circ, it follows that ∠ABC=∠ACB=180∘−∠BAC2\angle ABC = \angle ACB = \frac{180^\circ - \angle BAC}{2}.

2. **Configure Points D D and E E **:
Since EB=ED EB = ED , point E E is such that B B lies on the perpendicular bisector of segment ED ED .

3. Equal Angles Condition:
∠ABD≡∠CBE\angle ABD \equiv \angle CBE implies symmetry in the configuration. This suggests that segment BD BD is the angle bisector of ∠ABC\angle ABC, creating equal angles at these points with respect to the fixed angle from ∠CBE \angle CBE .

4. **Locate Point O O **:
O O is located at the intersection of the internal bisectors of angles ∠BDC\angle BDC and ∠ACB\angle ACB. Since these bisectors intersect, they form an incenter-like\textit{incenter-like} point for △BDC \triangle BDC .

5. **Calculate ∠COD\angle COD**:
With ∠BAC<60∘\angle BAC < 60^\circ, ∠CBD=∠ECB\angle CBD = \angle ECB and the structure where bisectors of complementary interior angles converge at point O O , it follows geometrically that:

∠COD=180∘−(∠BDC2+∠ACB2) \angle COD = 180^\circ - \left(\frac{\angle BDC}{2} + \frac{\angle ACB}{2}\right)

Given the symmetry and fixed conditions provided, it suggests placement where

∠COD≡120∘ \angle COD \equiv 120^\circ

Therefore, the desired angle ∠COD\angle COD is:

120∘ \boxed{120^\circ}

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Source: Omni-MATH, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.