Olympiad Maths Prep

Track / Stage 6 / 361 of 400 #1361 of 2000

Problem 1361

National olympiad, first round
Geometry Difficulty 6.8 Find the answer

Angle bisectors from vertices BB and CC and the perpendicular bisector of side BCBC are drawn in a non-isosceles triangle ABCABC. Next, three points of pairwise intersection of these three lines were marked (remembering which point is which), and the triangle itself was erased. Restore it according to the marked points using a compass and ruler.

(Yu. Blinkov)

Official solution

1. Identify the given lines and their properties:
- Let a a be the perpendicular bisector of side BC BC .
- Let b b be the angle bisector from vertex B B .
- Let c c be the angle bisector from vertex C C .

2. Reflect the angle bisectors about the perpendicular bisector:
- Construct line b b' by reflecting b b about a a .
- Construct line c c' by reflecting c c about a a .

3. **Determine the positions of vertices B B and C C :**
- Vertex B B is the intersection of lines c c' and b b .
- Vertex C C is the intersection of lines b b' and c c .

4. Construct the sides of the original triangle:
- Reflect side BC BC about the angle bisector b b to get a line. This line will be one of the sides of the original triangle.
- Reflect side BC BC about the angle bisector c c to get another line. This line will be another side of the original triangle.

5. **Find the intersection of the reflected lines to locate vertex A A :**
- The intersection of the two reflected lines (from step 4) will give the position of vertex A A .

6. **Reconstruct the original triangle ABC ABC :**
- Connect vertices A A , B B , and C C to form the triangle.

The original triangle ABC is restored. \boxed{\text{The original triangle } ABC \text{ is restored.}}

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