Maths Olympiad Prep

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Algebra Difficulty 5.1 AIME, harder Find the answer

6. How many solutions in integers does the equation

n(n+1)(n+2)(n+3)+m5=5000? n(n+1)(n+2)(n+3)+m^{5}=5000 ?

Solution

Answer: none.

Solution. Let for some natural numbers mm and nn the equality n(n+1)(n+2)(n+3)+m5=5000n(n+1)(n+2)(n+3)+m^{5}=5000 holds. Since n(n+1)(n+2)(n+3)=(n2+3n+1)21n(n+1)(n+2)(n+3)=\left(n^{2}+3 n+1\right)^{2}-1,
the equality can be rewritten as k2+m5=5001k^{2}+m^{5}=5001, where k=n2+3n+1k=n^{2}+3 n+1. Consider the remainders upon division by 11 of the left and right parts of the equality k2+m5=5001k^{2}+m^{5}=5001. Squares can only give remainders of 0,1,3,4,50,1,3,4,5 and 9, while fifth powers can only give remainders of 0,1, and 10. But no two of the written remainders add up to the remainder 6, which the number 5001 will have.

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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.