Maths Olympiad Prep

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Geometry Difficulty 7.3 National olympiad, round 2 Prove it

A circle cc with center AA passes through the vertices BB and EE of a regular pentagon ABCDEABCDE . The line BCBC intersects the circle cc for second time at point FF. The point GG on the circle cc is chosen such that FB=FG| F B | = | FG | and BGB \ne G. Prove that the lines AB,EFAB, EF and DGDG intersect at one point.

Solution

1. Identify the given elements and their properties:
- A circle c c with center A A passes through vertices B B and E E of a regular pentagon ABCDE ABCDE .
- The line BC BC intersects the circle c c for the second time at point F F .
- The point G G on the circle c c is chosen such that FB=FG |FB| = |FG| and BG B \ne G .

2. Establish the relationship between the points:
- Since FB=FG |FB| = |FG| , point G G is the reflection of point B B across the perpendicular bisector of F F on the circle c c .

3. Use the properties of the regular pentagon and circle:
- In a regular pentagon, the diagonals intersect at the golden ratio points. This implies that the segments AB AB , EF EF , and DG DG have specific symmetrical properties.

4. **Prove the concurrency of lines AB AB , EF EF , and DG DG :**
- Let X X be the intersection point of EF EF and DG DG .
- By the properties of the circle and the regular pentagon, we know that FXEX=FGDE=FBBC \frac{FX}{EX} = \frac{FG}{DE} = \frac{FB}{BC} .

5. Use the parallelism and intersection properties:
- Since CEAB CE \parallel AB and X X lies on AB AB , we can conclude that the lines AB AB , EF EF , and DG DG intersect at one point.

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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.