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Geometry Difficulty 6.3 National olympiad Find the answer

Let ABCDEFABCDEF be a regular hexagon with side 1. Point X,YX, Y are on sides CDCD and DEDE respectively, such that the perimeter of DXYDXY is 22. Determine XAY\angle XAY.

A number or a short expression. Spacing and $ signs are ignored.

Solution

To solve for XAY\angle XAY, we first establish the geometry of the problem. We have a regular hexagon ABCDEFABCDEF with side length 1. Since it is regular, each interior angle of the hexagon is 120120^\circ.

Points XX and YY are located on sides CDCD and DEDE, respectively, with the condition that the perimeter of triangle DXYDXY is 2. We label the distances DX=aDX = a and DY=bDY = b. Therefore, the perimeter condition can be written as:

1+a+b=2 1 + a + b = 2

Which simplifies to:

a+b=1 a + b = 1

Since we're dealing with a regular hexagon, we place it on the complex plane with center OO at the origin, such that the vertices A,B,C,D,E,FA, 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=eiπ/3 A = 1, \quad B = e^{i\pi/3}, \quad C = e^{i2\pi/3}, \quad D = -1, \quad E = e^{i4\pi/3}, \quad F = e^{-i\pi/3}

Point DD can be written as 1-1, so point XX on line segment CDCD can be parameterized as:
X=(1a)ei2π/3+a(1) X = (1-a)\cdot e^{i2\pi/3} + a \cdot (-1)
X=(1a)(12+32i)a X = (1-a)\left(-\frac{1}{2} + \frac{\sqrt{3}}{2}i\right) - a
X=a1a2+1a23i X = -a - \frac{1-a}{2} + \frac{1-a}{2}\sqrt{3} i

Similarly, point YY on segment DEDE can be parametrized:
Y=(1b)(1)+bei4π/3 Y = (1-b)\cdot(-1) + b \cdot e^{i4\pi/3}
Y=(1b)+b(1232i) Y = -(1-b) + b\left(-\frac{1}{2} - \frac{\sqrt{3}}{2}i\right)
Y=1+b+b(12)b(32i) Y = -1+b + b\left(-\frac{1}{2}\right) - b\left(\frac{\sqrt{3}}{2}i\right)
Y=1+b2+i(b32) Y = -1 + \frac{b}{2} + i \left(- \frac{b\sqrt{3}}{2}\right)

To find XAY\angle XAY, use the argument of complex numbers, as the angle is the argument of the complex number yx1x\frac{y-x}{1-x}.

Since a+b=1a + b = 1, to simplify, we can observe geometry properties due to symmetry of hexagon - X,D,YX, D, Y will form an isosceles triangle with XDY=120\angle XDY = 120^\circ due to interior angles of the hexagon. Using this symmetry, XAY=180120=60\angle XAY = 180^\circ - 120^\circ = 60^\circ.

However the setup of hexagon and the path of line show half this angle due to triangle placement is:
XAY=30 \angle XAY = 30^\circ

Thus, the exact XAY=30\angle XAY = \boxed{30^\circ}.

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