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Algebra Difficulty 5.8 AIME, harder Find the answer

Find all the integers pairs (x,y)(x, y) which satisfy the equation

x5y5=16xy x^{5}-y^{5}=16 x y

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

If one of x,yx, y is 0, the other has to be 0 too, and (x,y)=(0,0)(x, y)=(0,0) is one solution. If xy0x y \neq 0, let d=gcd(x,y)d=\operatorname{gcd}(x, y) and we write x=da,y=db,a,bZx=d a, y=d b, a, b \in \mathbb{Z} with (a,b)=1(a, b)=1. Then, the given equation is transformed into

d3a5d3b5=16ab d^{3} a^{5}-d^{3} b^{5}=16 a b

So, by the above equation, we conclude that ad3b5a \mid d^{3} b^{5} and thus ad3a \mid d^{3}. Similarly bd3b \mid d^{3}. Since (a,b)=1(a, b)=1, we get that abd3a b \mid d^{3}, so we can write d3=abrd^{3}=a b r with rZr \in \mathbb{Z}. Then, equation (1) becomes

abra5abrb5=16abr(a5b5)=16 \begin{aligned} a b r a^{5}-a b r b^{5} & =16 a b \Rightarrow \\ r\left(a^{5}-b^{5}\right) & =16 \end{aligned}

Therefore, the difference a5b5a^{5}-b^{5} must divide 16. Therefore, the difference a5b5a^{5}-b^{5} must divide 16. This means that

a5b5=±1,±2,±4,±8,±16 a^{5}-b^{5}= \pm 1, \pm 2, \pm 4, \pm 8, \pm 16

The smaller values of a5b5\left|a^{5}-b^{5}\right| are 1 or 2. Indeed, if a5b5=1\left|a^{5}-b^{5}\right|=1 then a=±1a= \pm 1 and b=0b=0 or a=0a=0 and b=±1b= \pm 1, a contradiction. If a5b5=2\left|a^{5}-b^{5}\right|=2, then a=1a=1 and b=1b=-1 or a=1a=-1 and b=1b=1. Then r=8r=-8, and d3=8d^{3}=-8 or d=2d=-2. Therefore, (x,y)=(2,2)(x, y)=(-2,2). If a5b5>2\left|a^{5}-b^{5}\right|>2 then, without loss of generality, let a>ba>b and a2a \geq 2. Putting a=x+1a=x+1 with x1x \geq 1, we have

a5b5=(x+1)5b5(x+1)5x5=5x4+10x3+10x2+5x+131 \begin{aligned} \left|a^{5}-b^{5}\right| & =\left|(x+1)^{5}-b^{5}\right| \\ & \geq\left|(x+1)^{5}-x^{5}\right| \\ & =\left|5 x^{4}+10 x^{3}+10 x^{2}+5 x+1\right| \geq 31 \end{aligned}

which is impossible. Thus, the only solutions are (x,y)=(0,0)(x, y)=(0,0) or (2,2)(-2,2).

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.