Maths Olympiad Prep

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Number theory Difficulty 5.6 AIME, harder Find the answer

4. 240 Find three natural numbers x,y,zx, y, z that satisfy the equation x3+y4=z5x^{3}+y^{4}=z^{5}, and determine whether the solution set of this equation in the set of natural numbers is finite or infinite.

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

Solution

[Solution] Since 224+224=2252^{24}+2^{24}=2^{25}, we have
(28)3+(26)4=(25)5\left(2^{8}\right)^{3}+\left(2^{6}\right)^{4}=\left(2^{5}\right)^{5}

That is, x=28=256;y=26=64,z=25=32x=2^{8}=256 ; y=2^{6}=64, z=2^{5}=32 is a set of solutions to the given equation.

On the other hand, if (x0,y0,z0)\left(x_{0}, y_{0}, z_{0}\right) is a set of solutions to the given equation in the set of natural numbers, then (k20x0,k15y0,k12z0)\left(k^{20} x_{0}, k^{15} y_{0}, k^{12} z_{0}\right) is also a solution, where kk is any natural number.

Therefore, the set of solutions to the given equation in the set of natural numbers is infinite.

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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.