GeometryDifficulty 6.3National olympiadFind the answer
Let ABCDEF be a regular hexagon with side 1. Point X,Y are on sides CD and DE respectively, such that the perimeter of DXY is 2. Determine ∠XAY.
A number or a short expression. Spacing and $ signs are ignored.
Solution
To solve for ∠XAY, we first establish the geometry of the problem. We have a regular hexagon ABCDEF with side length 1. Since it is regular, each interior angle of the hexagon is 120∘.
Points X and Y are located on sides CD and DE, respectively, with the condition that the perimeter of triangle DXY is 2. We label the distances DX=a and DY=b. Therefore, the perimeter condition can be written as:
1+a+b=2
Which simplifies to:
a+b=1
Since we're dealing with a regular hexagon, we place it on the complex plane with center O at the origin, such that the vertices A,B,C,D,E,F have complex coordinates in the form of: A=1,B=eiπ/3,C=ei2π/3,D=−1,E=ei4π/3,F=e−iπ/3
Point D can be written as −1, so point X on line segment CD can be parameterized as: X=(1−a)⋅ei2π/3+a⋅(−1) X=(1−a)(−21+23i)−a X=−a−21−a+21−a3i
Similarly, point Y on segment DE can be parametrized: Y=(1−b)⋅(−1)+b⋅ei4π/3 Y=−(1−b)+b(−21−23i) Y=−1+b+b(−21)−b(23i) Y=−1+2b+i(−2b3)
To find ∠XAY, use the argument of complex numbers, as the angle is the argument of the complex number 1−xy−x.
Since a+b=1, to simplify, we can observe geometry properties due to symmetry of hexagon - X,D,Y will form an isosceles triangle with ∠XDY=120∘ due to interior angles of the hexagon. Using this symmetry, ∠XAY=180∘−120∘=60∘.
However the setup of hexagon and the path of line show half this angle due to triangle placement is: ∠XAY=30∘
Thus, the exact ∠XAY=30∘.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.
Source: Omni-MATH,
licensed Apache-2.0.
Statement and solution reproduced as published; topic and difficulty added by this site.