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Geometry Difficulty 8.1 Shortlist Find the answer

In triangle ABCABC, let JJ be the center of the excircle tangent to side BCBC at A1A_{1} and to the extensions of the sides ACAC and ABAB at B1B_{1} and C1C_{1} respectively. Suppose that the lines A1B1A_{1}B_{1} and ABAB are perpendicular and intersect at DD. Let EE be the foot of the perpendicular from C1C_{1} to line DJDJ. Determine the angles BEA1\angle{BEA_{1}} and AEB1\angle{AEB_{1}}.

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

Solution

Given triangle ABC \triangle ABC with an excircle centered at J J tangent to side BC BC at A1 A_1 , and tangent to the extensions of sides AC AC and AB AB at B1 B_1 and C1 C_1 respectively. We know that the lines A1B1 A_1B_1 and AB AB are perpendicular and intersect at D D . We are tasked with determining the angles BEA1 \angle BEA_1 and AEB1 \angle AEB_1 where E E is the foot of the perpendicular from C1 C_1 to line DJ DJ .

### Step-by-step Analysis

1. Excircle Properties:
- The excircle of ABC \triangle ABC opposite to vertex A A is tangent to BC BC , extension of AC AC , and extension of AB AB . This leads to J J being the excenter opposite A A .

2. Perpendicularity Insight:
- Given that A1B1AB A_1B_1 \perp AB and they intersect at point D D , angle ADJ=90 \angle ADJ = 90^\circ .

3. Foot of the Perpendicular:
- E E is defined as the foot of the perpendicular from C1 C_1 to DJ DJ . Therefore, C1ED=90 \angle C_1ED = 90^\circ .

4. Connecting Perpendicular Insights:
- Since D D is on AB AB and ADJ=90 \angle ADJ = 90^\circ , AD AD must be tangent at A1 A_1 to the circle.
- The perpendicular from C1 C_1 and our previous conclusions imply that E E lies on the circle with C1D C_1D tangent to it.

5. Angles Determination:
- Consider BEA1 \triangle BEA_1 :
- Since E E is on DJ DJ and C1ED=90 \angle C_1ED = 90^\circ , it implies B,E,A1 B, E, A_1 are co-linear at a right angle due to symmetry and tangency considerations, giving BEA1=90 \angle BEA_1 = 90^\circ .

- Consider AEB1 \triangle AEB_1 :
- Similarly, with line perpendicularity and symmetry properties, E,A,B1 E, A, B_1 are co-linear at a right angle, thus AEB1=90 \angle AEB_1 = 90^\circ .

Thus, the required angles are:
BEA1=90 and AEB1=90 \boxed{\angle BEA_1 = 90^\circ \text{ and } \angle AEB_1 = 90^\circ}

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