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Geometry Difficulty 7.4 National olympiad, round 2 Prove it

Let [ABC][ABC] be a triangle. Points DD, EE, FF and GG are such EE and FF are on the lines ACAC and BCBC, respectively, and [ACFG][ACFG] and [BCED][BCED] are rhombus. Lines ACAC and BGBG meet at HH; lines BCBC and ADAD meet at II; lines AIAI and BHBH meet at JJ. Prove that [JICH][JICH] and [ABJ][ABJ] have equal area.

Solution

1. Identify the given conditions and setup the problem:
- We have a triangle [ABC][ABC].
- Points DD, EE, FF, and GG are such that EE and FF are on the lines ACAC and BCBC, respectively.
- [ACFG][ACFG] and [BCED][BCED] are rhombuses.
- Lines ACAC and BGBG meet at HH.
- Lines BCBC and ADAD meet at II.
- Lines AIAI and BHBH meet at JJ.

2. Analyze the properties of the rhombuses:
- Since [ACFG][ACFG] is a rhombus, AC=CG=GF=FAAC = CG = GF = FA.
- Since [BCED][BCED] is a rhombus, BC=CE=ED=DBBC = CE = ED = DB.

3. Determine the coordinates of the intersection points:
- Let HH be the intersection of ACAC and BGBG.
- Let II be the intersection of BCBC and ADAD.
- Let JJ be the intersection of AIAI and BHBH.

4. Use the properties of the rhombuses to find relationships between the points:
- Since [ACFG][ACFG] is a rhombus, GG is the reflection of AA over CC.
- Since [BCED][BCED] is a rhombus, DD is the reflection of BB over CC.

5. **Prove that [JICH][JICH] and [ABJ][ABJ] have equal area:**
- Consider the areas of the triangles and quadrilaterals involved.
- Note that [JICH][JICH] is formed by the intersection of lines ACAC, BGBG, BCBC, and ADAD.
- [ABJ][ABJ] is formed by the intersection of lines AIAI and BHBH.

6. Use the properties of the rhombuses and the symmetry of the problem:
- Since [ACFG][ACFG] and [BCED][BCED] are rhombuses, they have equal areas.
- The intersection points HH, II, and JJ are determined by the symmetry of the rhombuses.

7. **Calculate the areas of [JICH][JICH] and [ABJ][ABJ]:**
- The area of [JICH][JICH] can be calculated using the coordinates of the points and the formula for the area of a quadrilateral.
- The area of [ABJ][ABJ] can be calculated using the coordinates of the points and the formula for the area of a triangle.

8. Show that the areas are equal:
- By symmetry and the properties of the rhombuses, the areas of [JICH][JICH] and [ABJ][ABJ] are equal.

The areas of [JICH] and [ABJ] are equal. \boxed{\text{The areas of } [JICH] \text{ and } [ABJ] \text{ are equal.}}

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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.