Maths Olympiad Prep

Track / Stage 6 / 37 of 400 #1037 of 1964

Problem 1037

National olympiad, first round
Geometry Difficulty 6.0 Prove it

10.5. On the coordinate plane, a family of concentric circles centered at point M(2;3)M(\sqrt{2} ; \sqrt{3}) is considered. a) Is there a circle in this family that has two rational points? b) Prove that there exists a circle in this family, inside which (i.e., inside the disk) there are exactly 2014 integer points. (A rational (integer) point is a point with rational (respectively, integer) coordinates.)

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Official solution

Answer: a) will not be found. Solution. a) Suppose, to the contrary, that there exist two rational points M1(x1,y1)M_{1}\left(x_{1}, y_{1}\right) and M2(x2,y2)M_{2}\left(x_{2}, y_{2}\right) on the circle of the given family. Then (x12)2+(y13)2=(x22)2+(y23)2\left(x_{1}-\sqrt{2}\right)^{2}+\left(y_{1}-\sqrt{3}\right)^{2}=\left(x_{2}-\sqrt{2}\right)^{2}+\left(y_{2}-\sqrt{3}\right)^{2}. Therefore, 2(x1x2)2+2(y1y2)3=q2\left(x_{1}-x_{2}\right) \sqrt{2}+2\left(y_{1}-y_{2}\right) \sqrt{3}=q- a rational number. If (x1x2)(y1y2)0\left(x_{1}-x_{2}\right) \cdot\left(y_{1}-y_{2}\right) \neq 0, then squaring the last equality, we get that 6\sqrt{6} is a rational number, which is false. If one of the differences, for example, x1x2x_{1}-x_{2}, is 0, then with y1y2y_{1} \neq y_{2} we obtain a contradiction with the irrationality of 3\sqrt{3}. Therefore, two different rational points on the circle cannot exist. b) Notice that inside a small-radius circle, there are no integer points (we can take a radius less than the distance from MM to the nearest integer point A(1;2)A(1 ; 2)). On the other hand, if we take a sufficiently large radius (for example, greater than 3000), then inside the circle there will be more than 2014 points. Since, by part a), when the radius is gradually increased, the jump in the number of integer points occurs only by one, there must necessarily come a moment when there will be exactly 2014 points inside the circle.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.