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Number theory Difficulty 5.5 AIME, harder Prove it Estonia

Does there exist a positive integer nn such that
1950n+1934n=2024n? 1950^n + 1934^n = 2024^n?

Solution

Answer: No.

The numbers 19501950, 19341934, and 20242024 give remainders 44, 22, and 11, respectively, when divided by 77. Raising 44 to powers n=1,2,3,4,5,6,n = 1, 2, 3, 4, 5, 6, \dots results in remainders 4,2,1,4,2,1,4, 2, 1, 4, 2, 1, \dots, and raising 22 to the same powers results in remainders 2,4,1,2,4,1,2, 4, 1, 2, 4, 1, \dots. Thus, the remainders of the left side of the given equation for powers n=1,2,3,4,5,6,n = 1, 2, 3, 4, 5, 6, \dots are 6,6,2,6,6,2,6, 6, 2, 6, 6, 2, \dots. The remainder of the right side is 11 for every nn. The contradiction shows that there is no positive integer nn that satisfies the given equation.

Clearly n=1n = 1 is not suitable, because 1950+1934>20241950 + 1934 > 2024. It is also easy to verify that

19502+19342=3802500+3740356=7542856>4096576=20242. 1950^2 + 1934^2 = 3802500 + 3740356 = 7542856 > 4096576 = 2024^2.

For n3n \ge 3, there are no integer solutions to the equation an+bn=cna^n + b^n = c^n by Fermat's Last Theorem.

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