Maths Olympiad Prep

Track / Stage 6 / 313 of 400 #1313 of 1964

Problem 1313

National olympiad, first round
Geometry Difficulty 6.5 Find the answer

A triangle is cut by 33 cevians from its 33 vertices into 77 pieces: 44 triangles and 33 quadrilaterals. Determine if it is possible that all 33 quadrilaterals are inscribed.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Official solution

1. Assume the given conditions:
- We have a triangle ABC \triangle ABC .
- Three cevians AD,BE, AD, BE, and CF CF are drawn from vertices A,B, A, B, and C C respectively, intersecting at a common point G G .
- These cevians divide the triangle into 4 smaller triangles and 3 quadrilaterals.

2. Identify the quadrilaterals:
- The quadrilaterals formed are DGHB,EHIC, DGHB, EHIC, and AGIF AGIF .

3. Assume that all three quadrilaterals are cyclic:
- A quadrilateral is cyclic if its opposite angles sum to 180 180^\circ .

4. Analyze the cyclic quadrilaterals:
- For DGHB DGHB to be cyclic, DGH+DBH=180 \angle DGH + \angle DBH = 180^\circ .
- For EHIC EHIC to be cyclic, EHI+ECI=180 \angle EHI + \angle ECI = 180^\circ .
- For AGIF AGIF to be cyclic, AGI+AFI=180 \angle AGI + \angle AFI = 180^\circ .

5. Consider the implications for the triangles:
- If DGHB DGHB is cyclic, then DGH=DBH \angle DGH = \angle DBH .
- If EHIC EHIC is cyclic, then EHI=ECI \angle EHI = \angle ECI .
- If AGIF AGIF is cyclic, then AGI=AFI \angle AGI = \angle AFI .

6. **Prove that the triangles AEC,BDC, AEC, BDC, and ABF ABF are isosceles:**
- Since DGHB DGHB is cyclic, DGH=DBH \angle DGH = \angle DBH , implying BDC \triangle BDC is isosceles with BD=DC BD = DC .
- Since EHIC EHIC is cyclic, EHI=ECI \angle EHI = \angle ECI , implying AEC \triangle AEC is isosceles with AE=EC AE = EC .
- Since AGIF AGIF is cyclic, AGI=AFI \angle AGI = \angle AFI , implying ABF \triangle ABF is isosceles with AF=FB AF = FB .

7. Derive a contradiction:
- From the isosceles triangles, we have AC=CE AC = CE , BC=BD BC = BD , and AB=AF AB = AF .
- This implies AC=CE<BC=BD<AB=AF<AC AC = CE < BC = BD < AB = AF < AC , which is a contradiction because it implies a cyclic inequality that cannot hold true.

8. Conclude that the assumption is false:
- Since assuming all three quadrilaterals are cyclic leads to a contradiction, it is not possible for all three quadrilaterals to be inscribed.

\blacksquare

The final answer is False.

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