To solve the problem, we need to find all pairs (x,y) of positive integers such that x2y+x+y is divisible by xy2+y+7.
We start by considering the divisibility condition:
xy2+y+7∣x2y+x+y
This implies that there exists an integer k such that:
x2y+x+y=k(xy2+y+7)
Expanding the right-hand side, we get:
x2y+x+y=kxy2+ky+7k
Comparing coefficients from both sides, we equate terms:
1. x2y=kxy2
2. x+y=ky+7k
From the first equation, assuming y=0, we have:
x2=kxy⟹x2−kxy=0⟹x(x−ky)=0
Since x is a positive integer, it implies:
x=ky
Substituting x=ky into the second equation gives:
ky+y=ky+7k⟹y=7k
So, from the above steps, we have found the general solution:
x=ky,y=7k
Therefore, a pair of solutions can be expressed as (x,y)=(k⋅7k,7k)=(7k2,7k).
Finally, we check small cases for specific integer values:
- For y=1, the divisibility condition reduces to checking:
x2+x+1≡0(modx+8)
Which simplifies to checking x=11 and x=49 satisfies the conditions separately.
Thus, the solutions are:
- (x,y)=(11,1),
- (x,y)=(49,1),
- General solution (x,y)=(7t2,7t), where 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.
Hence, the complete set of solutions is:
(11,1),(49,1),(7t2,7t)