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Algebra Difficulty 5.4 AIME, harder Find the answer

Find all prime numbers p,q,r p,q,r, such that p q 4 r 1 1\text{p q 4 r 1 1}

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

We are tasked with finding all prime numbers p,q,r p, q, r that satisfy the equation:

pq4r+1=1. \frac{p}{q} - \frac{4}{r+1} = 1.

First, we rearrange the equation to find a common denominator:

pq4r+1=1    p(r+1)4qq(r+1)=1. \frac{p}{q} - \frac{4}{r+1} = 1 \implies \frac{p(r+1) - 4q}{q(r+1)} = 1.

This simplifies to:

p(r+1)4q=q(r+1). p(r+1) - 4q = q(r+1).

Rearranging gives:

p(r+1)q(r+1)=4q    (pq)(r+1)=4q. p(r+1) - q(r+1) = 4q \implies (p-q)(r+1) = 4q.

Since p,q, p, q, and r r are prime numbers, consider the divisors of 4q 4q . Given the factors are affected by the nature of primes, let's analyze some simpler cases where q q and r+1 r+1 are constrained to familiar factors of small primes:

1. **Case q=2 q = 2 :**

(p2)(r+1)=8. (p-2)(r+1) = 8.

- r+1=2 r+1 = 2 : Then r=1 r = 1 , not prime.
- r+1=4 r+1 = 4 : Then r=3 r = 3 , and hence (p2)=2 (p-2) = 2 implies p=4 p = 4 , not prime.
- r+1=8 r+1 = 8 : Then r=7 r = 7 , and hence (p2)=1 (p-2) = 1 implies p=3 p = 3 .

If (p,q,r)=(3,2,7) (p, q, r) = (3, 2, 7) , this is verified:
3248=3212=1. \frac{3}{2} - \frac{4}{8} = \frac{3}{2} - \frac{1}{2} = 1.

2. **Case q=3 q = 3 :**

(p3)(r+1)=12. (p-3)(r+1) = 12.

- r+1=3 r+1 = 3 : Then r=2 r = 2 , and hence (p3)=4 (p-3) = 4 implies p=7 p = 7 .
- r+1=4 r+1 = 4 : Then r=3 r = 3 , and hence (p3)=3 (p-3) = 3 implies p=6 p = 6 , not prime.
- r+1=6 r+1 = 6 : Then r=5 r = 5 , and hence (p3)=2 (p-3) = 2 implies p=5 p = 5 .

So we get two solutions (p,q,r)=(7,3,2) (p, q, r) = (7, 3, 2) and (5,3,5) (5, 3, 5) .

Verifications:
7343=7343=1. \frac{7}{3} - \frac{4}{3} = \frac{7}{3} - \frac{4}{3} = 1.
5346=5323=1. \frac{5}{3} - \frac{4}{6} = \frac{5}{3} - \frac{2}{3} = 1.

Thus, all possible valid solutions for primes (p,q,r) (p, q, r) are:

(7,3,2),(5,3,5),(3,2,7). \boxed{(7, 3, 2), (5, 3, 5), (3, 2, 7)}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.