Olympiad Maths Prep

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Problem 1637

National olympiad second round; IMO P1/P4
Geometry Difficulty 7.5 Find the answer

Can a regular triangle be placed inside a regular hexagon in such a way that all vertices of the triangle were seen from each vertex of the hexagon? (Point AA is seen from BB, if the segment ABAB dots not contain internal points of the triangle.)

This one wants a proof. Work it on paper, read the official solution, then mark yourself honestly — the ladder only means something if the record is true.

Official solution

1. Assume the vertices of the equilateral triangle are at specific coordinates:
- Let the vertices of the equilateral triangle be A(1,0) A(1,0) , B(1,0) B(-1,0) , and C(0,3) C\left(0, \sqrt{3}\right) .

2. Determine the regions where the vertices of the hexagon can be:
- The vertices of the regular hexagon V1,V2,V3,V4,V5,V6 V_1, V_2, V_3, V_4, V_5, V_6 can only be in the three triangular regions A \mathcal{A} , B \mathcal{B} , and C \mathcal{C} formed by extending the sides of the equilateral triangle.

3. **Claim 0: There exists at least one vertex Vk V_k ( 1k6 1 \leq k \leq 6 ) in each of A \mathcal{A} , B \mathcal{B} , C \mathcal{C} :**
- Assume otherwise, and let there be no vertex in region A \mathcal{A} . Then the hexagon V1V2V3V4V5V6 V_1V_2V_3V_4V_5V_6 is entirely to the right of line BC BC , and thus ABC \triangle ABC isn't inside V1V2V3V4V5V6 V_1V_2V_3V_4V_5V_6 . This is a contradiction.

4. **Claim 1: Points Vi(x1,y1) V_i(x_1, y_1) and Vi+3(x2,y2) V_{i+3}(x_2, y_2) cannot be in the same region:**
- Assume otherwise, and let them both be in C \mathcal{C} . So y13±3x1 y_1 \geq \sqrt{3} \pm \sqrt{3}x_1 and y23±3x2 y_2 \geq \sqrt{3} \pm \sqrt{3}x_2 .
- By complex numbers, Vi+2(x1+3x23y1+3y24,3x13x2+y1+3y24) V_{i+2} \left( \frac{x_1 + 3x_2 - \sqrt{3}y_1 + \sqrt{3}y_2}{4}, \frac{\sqrt{3}x_1 - \sqrt{3}x_2 + y_1 + 3y_2}{4} \right) .
- Since 3x13x2+y1+3y2>y1+3x13+y23x230 \sqrt{3}x_1 - \sqrt{3}x_2 + y_1 + 3y_2 > y_1 + \sqrt{3}x_1 - \sqrt{3} + y_2 - \sqrt{3}x_2 - \sqrt{3} \geq 0 , Vi+2 V_{i+2} is above the line AB AB and therefore can only be in C \mathcal{C} .
- We can similarly prove that each of V1,V2,V3,V4,V5,V6 V_1, V_2, V_3, V_4, V_5, V_6 is inside C \mathcal{C} , which contradicts Claim 0.

5. **Claim 2: If Vi(x1,y1) V_i(x_1, y_1) is in the same region as Vi+2(x2,y2) V_{i+2}(x_2, y_2) , then so are Vi+1 V_{i+1} and Vi+4 V_{i+4} :**
- Let Vi V_i and Vi+2 V_{i+2} be both in C \mathcal{C} . So y13±3x1 y_1 \geq \sqrt{3} \pm \sqrt{3}x_1 and y23±3x2 y_2 \geq \sqrt{3} \pm \sqrt{3}x_2 .
- By complex numbers, Vi+1(3x1+3x2y1+y223,x1x2+3y1+3y223) V_{i+1} \left( \frac{\sqrt{3}x_1 + \sqrt{3}x_2 - y_1 + y_2}{2\sqrt{3}}, \frac{x_1 - x_2 + \sqrt{3}y_1 + \sqrt{3}y_2}{2\sqrt{3}} \right) .
- Since x1x2+3y1+3y2>y1+3x133+y23x2330 x_1 - x_2 + \sqrt{3}y_1 + \sqrt{3}y_2 > \frac{y_1 + \sqrt{3}x_1 - \sqrt{3}}{\sqrt{3}} + \frac{y_2 - \sqrt{3}x_2 - \sqrt{3}}{\sqrt{3}} \geq 0 , Vi+1 V_{i+1} is above the line AB AB and therefore can only be in C \mathcal{C} .
- Additionally, Vi+4(x1+x2+3y13y22,y1+y23x1+3x22) V_{i+4} \left( \frac{x_1 + x_2 + \sqrt{3}y_1 - \sqrt{3}y_2}{2}, \frac{y_1 + y_2 - \sqrt{3}x_1 + \sqrt{3}x_2}{2} \right) , and since y1+y23x1+3x2>y13x13+y2+3x230 y_1 + y_2 - \sqrt{3}x_1 + \sqrt{3}x_2 > y_1 - \sqrt{3}x_1 - \sqrt{3} + y_2 + \sqrt{3}x_2 - \sqrt{3} \geq 0 , Vi+4 V_{i+4} is above the line AB AB and therefore can only be in C \mathcal{C} .

6. **Consider the multiset S={#{1i6:ViA},#{1i6:ViB},#{1i6:ViC}} S = \left\{ \#\{1 \leq i \leq 6 : V_i \in \mathcal{A}\}, \#\{1 \leq i \leq 6 : V_i \in \mathcal{B}\}, \#\{1 \leq i \leq 6 : V_i \in \mathcal{C}\} \right\} :**
- Each element of S S has to be greater than or equal to one (by Claim 0) and less than or equal to three (otherwise by the pigeonhole principle, there exists a pair of opposite vertices both chosen, which yields a contradiction with Claim 1).
- We therefore have the following possibilities: {1,2,3} \{1, 2, 3\} and {2,2,2} \{2, 2, 2\} .

7. Analyze the possibilities:
- For {1,2,3} \{1, 2, 3\} , Claim 2 says that the "2" must be adjacent, say V1 V_1 and V2 V_2 . But by Claim 2, the pairs (V4,V6) (V_4, V_6) and (V3,V5) (V_3, V_5) can't be in the same region; otherwise, the equilateral V1V3V5 \triangle V_1V_3V_5 or V2V4V6 V_2V_4V_6 yields a contradiction with Claim 2.
- So the only possibility is S={2,2,2} S = \{2, 2, 2\} , from which the solution is similar to StarChan's.

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.