Maths Olympiad Prep

Library / /1 of 4

Number theory Difficulty 6.9 National olympiad Prove it Asia Pacific Mathematics Olympiad (APMO)

Find all positive integers aa and bb such that
a2+bb2a and b2+aa2b \frac{a^{2}+b}{b^{2}-a} \text{ and } \frac{b^{2}+a}{a^{2}-b}
are both integers.

Solution

By the symmetry of the problem, we may suppose that aba \leq b. Notice that b2a0b^{2}-a \geq 0, so that if a2+bb2a\frac{a^{2}+b}{b^{2}-a} is a positive integer, then a2+bb2aa^{2}+b \geq b^{2}-a. Rearranging this inequality and factorizing, we find that (a+b)(ab+1)0(a+b)(a-b+1) \geq 0. Since a,b>0a, b>0, we must have ab1a \geq b-1.

We therefore have two cases:

Case 1: a=ba=b. Substituting, we have
a2+aa2a=a+1a1=1+2a1 \frac{a^{2}+a}{a^{2}-a}=\frac{a+1}{a-1}=1+\frac{2}{a-1}
which is an integer if and only if (a1)2(a-1) \mid 2. As a>0a>0, the only possible values are a1=1a-1=1 or 22. Hence, (a,b)=(2,2)(a, b)=(2,2) or (3,3)(3,3).

Case 2: a=b1a=b-1. Substituting, we have
b2+aa2b=(a+1)2+aa2(a+1)=a2+3a+1a2a1=1+4a+2a2a1. \frac{b^{2}+a}{a^{2}-b}=\frac{(a+1)^{2}+a}{a^{2}-(a+1)}=\frac{a^{2}+3 a+1}{a^{2}-a-1}=1+\frac{4 a+2}{a^{2}-a-1}.
Once again, notice that 4a+2>04 a+2>0, and hence, for 4a+2a2a1\frac{4 a+2}{a^{2}-a-1} to be an integer, we must have 4a+2a2a14 a+2 \geq a^{2}-a-1, that is, a25a30a^{2}-5 a-3 \leq 0. Hence, since aa is an integer, we can bound aa by 1a51 \leq a \leq 5. Checking all the ordered pairs (a,b)=(1,2),(2,3),,(5,6)(a, b)=(1,2),(2,3), \ldots,(5,6), we find that only (1,2)(1,2) and (2,3)(2,3) satisfy the given conditions.

Thus, the ordered pairs that work are
(2,2),(3,3),(1,2),(2,3),(2,1),(3,2) (2,2),(3,3),(1,2),(2,3),(2,1),(3,2)
where the last two pairs follow by symmetry.

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: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.