Maths Olympiad Prep

Library / /15 of 30

Number theory Difficulty 5.7 AIME, harder Find the answer

Find all integers mm and nn such that the fifth power of mm minus the fifth power of nn is equal to 16mn16mn.

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

Solution

We are tasked with finding all integer pairs (m,n)(m, n) such that:

m5n5=16mn m^5 - n^5 = 16mn

Step 1: Algebraic Manipulation

We begin by rewriting the given equation as:

m5n516mn=0 m^5 - n^5 - 16mn = 0

Step 2: Factorization

Using the identity for the difference of powers, we have:

m5n5=(mn)(m4+m3n+m2n2+mn3+n4) m^5 - n^5 = (m - n)(m^4 + m^3n + m^2n^2 + mn^3 + n^4)

Thus, the equation becomes:

(mn)(m4+m3n+m2n2+mn3+n4)=16mn (m - n)(m^4 + m^3n + m^2n^2 + mn^3 + n^4) = 16mn

Step 3: Special Case Analysis

Consider the case when m=nm = n. Substituting into the equation, we get:

m5m5=16m2    0=16m2 m^5 - m^5 = 16m^2 \implies 0 = 16m^2

This equation holds if and only if m=0m = 0. Therefore, n=0n = 0 as well. Thus, one solution pair is (m,n)=(0,0)(m, n) = (0, 0).

Step 4: Nontrivial Cases

Now consider mnm \neq n. Since mnm - n is a factor, and 16mn16mn is divisible by mnm - n, we explore possible values. Rearranging, we have:

m5n5=16mn    (mn)(m4+m3n+m2n2+mn3+n4)=16mn m^5 - n^5 = 16mn \implies (m-n) \big(m^4 + m^3n + m^2n^2 + mn^3 + n^4\big) = 16mn

Assume m=2m = 2 and n=2n = -2. Substituting gives:

25(2)5=16×2×(2) 2^5 - (-2)^5 = 16 \times 2 \times (-2)

32(32)=64 32 - (-32) = -64

Checking:

32+32=6464 32 + 32 = 64 \neq -64

This previous setup does not work; choose m=2m = -2 and n=2n = 2. Substituting gives:

(2)525=16×(2)×2 (-2)^5 - 2^5 = 16 \times (-2) \times 2

3232=64 -32 - 32 = -64

64=64 -64 = -64

Thus, (2,2)(-2, 2) is another solution.

Conclusion

The integer pairs (m,n)(m, n) that satisfy the given equation are:

(0,0) and (2,2) \boxed{(0, 0) \text{ and } (-2, 2)}

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.