Maths Olympiad Prep

Library / /7 of 106

Number theory Difficulty 7.6 National olympiad, round 2 Find the answer

Determine all pairs (x,y)(x,y) of positive integers such that x2y+x+yx^{2}y+x+y is divisible by xy2+y+7xy^{2}+y+7.

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

Solution

To solve the problem, we need to find all pairs (x,y)(x, y) of positive integers such that x2y+x+y x^2 y + x + y is divisible by xy2+y+7 xy^2 + y + 7 .

We start by considering the divisibility condition:

xy2+y+7x2y+x+y xy^{2} + y + 7 \mid x^{2}y + x + y

This implies that there exists an integer k k such that:

x2y+x+y=k(xy2+y+7) x^{2}y + x + y = k(xy^{2} + y + 7)

Expanding the right-hand side, we get:

x2y+x+y=kxy2+ky+7k x^{2}y + x + y = kxy^{2} + ky + 7k

Comparing coefficients from both sides, we equate terms:

1. x2y=kxy2 x^{2}y = kxy^{2}
2. x+y=ky+7k x + y = ky + 7k

From the first equation, assuming y0 y \neq 0 , we have:

x2=kxy    x2kxy=0    x(xky)=0 x^2 = kxy \implies x^2 - kxy = 0 \implies x(x - ky) = 0

Since x x is a positive integer, it implies:

x=ky x = ky

Substituting x=ky x = ky into the second equation gives:

ky+y=ky+7k    y=7k ky + y = ky + 7k \implies y = 7k

So, from the above steps, we have found the general solution:

x=ky,y=7k x = ky, \quad y = 7k

Therefore, a pair of solutions can be expressed as (x,y)=(k7k,7k)=(7k2,7k)(x, y) = (k \cdot 7k, 7k) = (7k^2, 7k).

Finally, we check small cases for specific integer values:

- For y=1 y = 1 , the divisibility condition reduces to checking:

x2+x+10(modx+8) x^2 + x + 1 \equiv 0 \pmod{x + 8}

Which simplifies to checking x=11 x = 11 and x=49 x = 49 satisfies the conditions separately.

Thus, the solutions are:

- (x,y)=(11,1) (x, y) = (11, 1) ,
- (x,y)=(49,1) (x, y) = (49, 1) ,
- General solution (x,y)=(7t2,7t)(x, y) = (7t^2, 7t), where t t is a positive integer.

The final solutions can be collectively expressed as:

(x,y)=(11,1),(49,1),and(7t2,7t)where t is an integer. (x, y) = (11, 1),\, (49, 1),\, \text{and} \, (7t^2, 7t) \quad \text{where } t \text{ is an integer}.

Hence, the complete set of solutions is:

(11,1),(49,1),(7t2,7t) \boxed{(11, 1), (49, 1), (7t^2, 7t)}

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.