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Geometry Difficulty 5.8 AIME, harder Prove it Croatia

Let ABCABC be an equilateral triangle with sides of length 11. The point XX on the ray ABAB and the point YY on the ray ACAC are chosen, so that AX|AX| and AY|AY| are positive integers.
Can the radius of the circumcircle of the triangle AXYAXY be 2014\sqrt{2014}?

Solution

Let us assume that the radius of the circumcircle of the triangle AXYAXY is equal to 2014\sqrt{2014}.
In the triangle AXYAXY the angle opposite to the side XYXY is 6060^\circ because the triangle ABCABC is equilateral.
If RR is the radius of the circumcircle of the triangle AXYAXY, then
XY=2Rsin60=20143. |XY| = 2R \sin 60^\circ = \sqrt{2014} \cdot \sqrt{3}.
Let AX=m|AX| = m, AY=n|AY| = n, where mm and nn are positive integers.
Figure 1
Applying the cosine rule to the triangle AXYAXY we get m2+n22mncos60=XY2m^2 + n^2 - 2mn \cos 60^\circ = |XY|^2, which leads to
m2+n2mn=20143. m^2 + n^2 - mn = 2014 \cdot 3.
Let us first notice that the right-hand side of the equation is even. If mm and nn are of different parity or if they are both odd, then the left-hand side is odd, which is impossible.
The only remaining case is when mm and nn are both even. Then m2+n2mnm^2 + n^2 - mn is divisible by 44, but 201432014 \cdot 3 is not, so we get another impossible case.

Therefore, the initial assumption is wrong, which means that RR can not be equal to 2014\sqrt{2014}.

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