Maths Olympiad Prep

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Geometry Difficulty 6.4 National olympiad Find the answer

Let ABCABC be a triangle and II its incenter. Suppose AI=2AI=\sqrt{2}, BI=5BI=\sqrt{5}, CI=10CI=\sqrt{10} and the inradius is 11. Let AA' be the reflection of II across BCBC, BB' the reflection across ACAC, and CC' the reflection across ABAB. Compute the area of triangle ABCA'B'C'.

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Identify the points of tangency and reflections:
Let D,E,FD, E, F be the points of tangency of the incircle with sides BC,AC,BC, AC, and ABAB respectively. The points A,B,CA', B', C' are the reflections of the incenter II across the sides BC,AC,BC, AC, and ABAB respectively.

2. Similarity of triangles:
Note that triangle ABCA'B'C' is similar to triangle DEFDEF with a ratio of 2:12:1. This is because the reflections of the incenter II across the sides of the triangle form a triangle that is homothetic to the contact triangle DEFDEF with a homothety ratio of 22.

3. **Area of triangle DEFDEF:**
To find the area of triangle DEFDEF, we first need to compute the area of the smaller triangles EFI,DFI,EFI, DFI, and DEIDEI.

4. **Area of triangle EFIEFI:**
By the Pythagorean theorem, we have:
AF=AE=AI2FI2=21=1 AF = AE = \sqrt{AI^2 - FI^2} = \sqrt{2 - 1} = 1
Let GG be the intersection of AIAI and EFEF. Since triangle FGIFGI is similar to triangle AFIAFI, we have:
FGAF=FIAI    FG1=12    FG=12 \frac{FG}{AF} = \frac{FI}{AI} \implies \frac{FG}{1} = \frac{1}{\sqrt{2}} \implies FG = \frac{1}{\sqrt{2}}
Similarly,
GIFI=FIAI    GI1=12    GI=12 \frac{GI}{FI} = \frac{FI}{AI} \implies \frac{GI}{1} = \frac{1}{\sqrt{2}} \implies GI = \frac{1}{\sqrt{2}}
Therefore, the area of triangle EFIEFI is:
Area of EFI=12EFGI=122212=12 \text{Area of } EFI = \frac{1}{2} \cdot EF \cdot GI = \frac{1}{2} \cdot \frac{2}{\sqrt{2}} \cdot \frac{1}{\sqrt{2}} = \frac{1}{2}

5. **Area of triangles DFIDFI and DEIDEI:**
By similar calculations, we find the areas of triangles DFIDFI and DEIDEI:
Area of DFI=25,Area of DEI=310 \text{Area of } DFI = \frac{2}{5}, \quad \text{Area of } DEI = \frac{3}{10}

6. **Total area of triangle DEFDEF:**
Summing the areas of the three smaller triangles, we get:
Area of DEF=12+25+310=65 \text{Area of } DEF = \frac{1}{2} + \frac{2}{5} + \frac{3}{10} = \frac{6}{5}

7. **Area of triangle ABCA'B'C':**
Since triangle ABCA'B'C' is similar to triangle DEFDEF with a ratio of 2:12:1, the area of ABCA'B'C' is:
Area of ABC=4×Area of DEF=4×65=245 \text{Area of } A'B'C' = 4 \times \text{Area of } DEF = 4 \times \frac{6}{5} = \frac{24}{5}

The final answer is 245\boxed{\frac{24}{5}}

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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.