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Number theory Difficulty 5.6 AIME, harder Prove it North Macedonia

Find all prime numbers p,q,rp, q, r, such that pq4r+1=1\frac{p}{q} - \frac{4}{r+1} = 1.

Solution

(p,q,r){(3,2,7),(5,3,5),(7,3,2)}(p, q, r) \in \{(3, 2, 7), (5, 3, 5), (7, 3, 2)\}

We can rewrite in the form
pr+p4qq(r+1)=1pr+p4q=qr+qr(pq)=5qp. \frac{pr + p - 4q}{q(r + 1)} = 1 \Rightarrow pr + p - 4q = qr + q \Rightarrow r(p - q) = 5q - p.

Hence pqp \neq q
r=5qppq=4q+qppqi.e.r=4qpq1. r = \frac{5q-p}{p-q} = \frac{4q+q-p}{p-q} \quad \text{i.e.} \quad r = \frac{4q}{p-q} - 1.
So pqqp-q \neq q, pq2qp-q \neq 2q, pq4qp-q \neq 4q. We have pq=1p-q=1 or pq=2p-q=2 or pq=4p-q=4.

i) If pq=1p-q=1 then q=2,p=3,r=7q=2, p=3, r=7.

ii) If pq=2p-q=2 then p=q+2,r=2q1p=q+2, r=2q-1.
If q1(mod3)q \equiv 1 \pmod{3} then q+20(mod3)q+2 \equiv 0 \pmod{3}, q+2=3q=1q+2=3 \Rightarrow q=1 contradiction.
If q1(mod3)q \equiv -1 \pmod{3} then r210(mod3)r \equiv -2-1 \equiv 0 \pmod{3}, so r=3,q=2,p=4r=3, q=2, p=4 contradiction.
Hence q=3,p=5,r=5q=3, p=5, r=5.

iii) If pq=4p-q=4 then p=q+4,r=q1p=q+4, r=q-1. Hence q=3,p=7,r=2q=3, p=7, r=2.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.