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Number theory Difficulty 6.2 National Olympiad Prove it Ukraine

Out of three expressions xy\frac{x}{y}, x2+xy2+y\frac{x^2+x}{y^2+y} and x2+2y2+2\frac{x^2+2}{y^2+2}, for some integer x,yx, y all three are defined, two take the same integer value, and the remaining one takes a different integer value. For which pairs of integers x,yx, y is this possible?

(Bohdan Rublyov)

Solution

Obviously, x>y|x| > |y|, for x0x \neq 0, since otherwise, the first fraction will not be an integer. If the first two expressions are equal, then we have the equality:
xy=x2+xy2+yxy2+xy=yx2+xyxy(yx)=0. \frac{x}{y} = \frac{x^2+x}{y^2+y} \Rightarrow xy^2 + xy = yx^2 + xy \Rightarrow xy(y-x) = 0.
For xyx \neq y and possible range of values, the given equality is satisfied only by pairs (x,y)=(0,t)(x, y) = (0, t), t0;1t \neq 0; -1. The third expression will obviously take a non-zero, but non-integer value, since 0<2t2+2<10 < \frac{2}{t^2+2} < 1 for t0t \neq 0.

If the first and third expressions are equal, we have the equality:
xy=x2+2y2+2xy2+2x=yx2+2yxy(yx)=2(yx)xy=2, \frac{x}{y} = \frac{x^2+2}{y^2+2} \Rightarrow xy^2 + 2x = yx^2 + 2y \Rightarrow xy(y-x) = 2(y-x) \Rightarrow xy = 2,
The possible pairs of (x,y)(x, y) are (2, 1) and (-2, -1). Here, the second expression in the first case will take on a value 3, hence, different from the other two. In the second case, it is not defined and, therefore, not possible.

If the second and third expressions are equal, we have the equality:
x2+xy2+y=x2+2y2+2x2y2+xy2+2x2+2x=x2y2+yx2+2y2+2yxy(yx)=2(yx)(y+x)+2(yx)xy2y2x=2(x2)(y2)=6. \frac{x^2+x}{y^2+y} = \frac{x^2+2}{y^2+2} \Rightarrow x^2y^2 + xy^2 + 2x^2 + 2x = x^2y^2 + yx^2 + 2y^2 + 2y \Rightarrow \\ xy(y-x) = 2(y-x)(y+x) + 2(y-x) \Rightarrow xy - 2y - 2x = 2 \Rightarrow (x-2)(y-2) = 6.

x2=6,y2=1x=8,y=3x2+2y2+2=6611=6x-2=6, y-2=1 \Rightarrow x=8, y=3 \Rightarrow \frac{x^2+2}{y^2+2} = \frac{66}{11} = 6 and xy=83\frac{x}{y} = \frac{8}{3} does not satisfy the conditions.

x2=3,y2=2x=5,y=4x2+2y2+2=2718x-2=3, y-2=2 \Rightarrow x=5, y=4 \Rightarrow \frac{x^2+2}{y^2+2} = \frac{27}{18} is not integer, so does not satisfy the conditions.
x2=2,y2=3x=0,y=1x-2=-2, y-2=-3 \Rightarrow x=0, y=-1 does not lie in the allowed range of values.
x2=6,y2=1x=4,y=1x2+2y2+2=183=6x-2=-6, y-2=-1 \Rightarrow x=-4, y=1 \Rightarrow \frac{x^2+2}{y^2+2} = \frac{18}{3} = 6 and xy=4\frac{x}{y} = -4 satisfies the conditions.

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