Maths Olympiad Prep

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Geometry Difficulty 6.4 National olympiad Find the answer

Let ABCABC be a triangle with incenter II and AB=1400AB = 1400, AC=1800AC = 1800, BC=2014BC = 2014. The circle centered at II passing through AA intersects line BCBC at two points XX and YY. Compute the length XYXY.

Proposed by Evan Chen

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

Solution

1. Define the given parameters and setup the problem:
- Let ABCABC be a triangle with incenter II.
- Given side lengths: AB=1400AB = 1400, AC=1800AC = 1800, BC=2014BC = 2014.
- The circle centered at II passing through AA intersects line BCBC at points XX and YY.
- We need to compute the length XYXY.

2. **Calculate the semi-perimeter ss of ABC\triangle ABC:**
s=AB+AC+BC2=1400+1800+20142=2607 s = \frac{AB + AC + BC}{2} = \frac{1400 + 1800 + 2014}{2} = 2607

3. **Determine the length ADAD where DD is the foot of the altitude from II to ABAB:**
AD=sa=26072014=593 AD = s - a = 2607 - 2014 = 593

4. **Express the radius RR of the circle centered at II passing through AA:**
R2=AD2+r2=5932+r2 R^2 = AD^2 + r^2 = 593^2 + r^2

5. **Calculate the power of point CC with respect to the circle ω\omega:**
- Let EE be the foot of the altitude from II to BCBC.
- The power of CC is given by:
CI2R2=CXCY=CY(CY+XY) CI^2 - R^2 = CX \cdot CY = CY(CY + XY)
- We know:
CI2=CE2+r2=12072+r2 CI^2 = CE^2 + r^2 = 1207^2 + r^2
- Therefore:
12072+r2(5932+r2)=120725932 1207^2 + r^2 - (593^2 + r^2) = 1207^2 - 593^2

6. Simplify the expression:
120725932=(1207593)(1207+593)=6141800 1207^2 - 593^2 = (1207 - 593)(1207 + 593) = 614 \cdot 1800

7. **Calculate CYCY:**
- Since CY=CEYZ=1207YZ=1207R2r2=1207593=614CY = CE - YZ = 1207 - YZ = 1207 - \sqrt{R^2 - r^2} = 1207 - 593 = 614.

8. **Determine XYXY:**
614(614+XY)=6141800 614 \cdot (614 + XY) = 614 \cdot 1800
614+XY=1800 614 + XY = 1800
XY=1800614=1186 XY = 1800 - 614 = 1186

The final answer is 1186\boxed{1186}

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.