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 A is seen from B, if the segment AB 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), B(−1,0), and C(0,3).
2. Determine the regions where the vertices of the hexagon can be: - The vertices of the regular hexagon V1,V2,V3,V4,V5,V6 can only be in the three triangular regions A, B, and C formed by extending the sides of the equilateral triangle.
3. **Claim 0: There exists at least one vertex Vk ( 1≤k≤6 ) in each of A, B, C:** - Assume otherwise, and let there be no vertex in region A. Then the hexagon V1V2V3V4V5V6 is entirely to the right of line BC, and thus △ABC isn't inside V1V2V3V4V5V6. This is a contradiction.
4. **Claim 1: Points Vi(x1,y1) and Vi+3(x2,y2) cannot be in the same region:** - Assume otherwise, and let them both be in C. So y1≥3±3x1 and y2≥3±3x2. - By complex numbers, Vi+2(4x1+3x2−3y1+3y2,43x1−3x2+y1+3y2). - Since 3x1−3x2+y1+3y2>y1+3x1−3+y2−3x2−3≥0, Vi+2 is above the line AB and therefore can only be in C. - We can similarly prove that each of V1,V2,V3,V4,V5,V6 is inside C, which contradicts Claim 0.
5. **Claim 2: If Vi(x1,y1) is in the same region as Vi+2(x2,y2), then so are Vi+1 and Vi+4:** - Let Vi and Vi+2 be both in C. So y1≥3±3x1 and y2≥3±3x2. - By complex numbers, Vi+1(233x1+3x2−y1+y2,23x1−x2+3y1+3y2). - Since x1−x2+3y1+3y2>3y1+3x1−3+3y2−3x2−3≥0, Vi+1 is above the line AB and therefore can only be in C. - Additionally, Vi+4(2x1+x2+3y1−3y2,2y1+y2−3x1+3x2), and since y1+y2−3x1+3x2>y1−3x1−3+y2+3x2−3≥0, Vi+4 is above the line AB and therefore can only be in C.
6. **Consider the multiset S={#{1≤i≤6:Vi∈A},#{1≤i≤6:Vi∈B},#{1≤i≤6:Vi∈C}}:** - Each element of 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} and {2,2,2}.
7. Analyze the possibilities: - For {1,2,3}, Claim 2 says that the "2" must be adjacent, say V1 and V2. But by Claim 2, the pairs (V4,V6) and (V3,V5) can't be in the same region; otherwise, the equilateral △V1V3V5 or V2V4V6 yields a contradiction with Claim 2. - So the only possibility is 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.