2015 AIME I Problem 3

Below is the professionally curated solution for Problem 3 of the 2015 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2015 AIME I solutions, or check the answer key.

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Concepts:sum and difference of cubesprimeparity

Difficulty rating: 2010

3.

There is a prime number pp such that 16p+116p + 1 is the cube of a positive integer. Find p.p.

Solution:

Write 16p+1=n3,16p + 1 = n^3, so 16p=n31=(n1)(n2+n+1).16p = n^3 - 1 = (n - 1)(n^2 + n + 1). Since 16p+116p + 1 is odd, nn is odd, and n2+n+1n^2 + n + 1 is odd as well. Therefore all four factors of 22 must divide n1:n - 1: write n1=16k,n - 1 = 16k, which gives p=k(n2+n+1).p = k(n^2 + n + 1). For pp to be prime we need k=1,k = 1, so n=17.n = 17.

Then p=172+17+1=307,p = 17^2 + 17 + 1 = 307, which is indeed prime, and 16307+1=4913=173.16 \cdot 307 + 1 = 4913 = 17^3.

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