Try this beautiful problem from the PRMO, 2019 based on Smallest Positive Integer.

## Smallest Positive Integer – PRMO 2019

Find the smallest positive integer n\(\geq\)10 such that n+6 is a prime and 9n+7 is a perfect square.

- is 107
- is 53
- is 840
- cannot be determined from the given information

**Key Concepts**

Integers

Primes

Perfect Square

## Check the Answer

But try the problem first…

Answer: is 53.

PRMO, 2019, Question 14

Elementary Number Theory by David Burton

## Try with Hints

First hint

Let 9n+7=\(m^{2}\) n+6 prime then n+6 odd then n is odd then n=2k+1 then 9(2k+1)+7=\(m^{2}\) then 18k=\(m^{2}\)-16=(m+4)(m-4) then 18k even m is even then m=2p

Second Hint

18k=(2p+4)(2p-4)=4(p+2)(p-2) then 9k=2(p+2)(p-2)then k even then k=2d then 18d=2(p+2)(p-2) then 9d=(p+2)(p-2) then p of form 9q+2,9q-2

Final Step

for p=9q-2 then m=2(9q-2) for q=1 then\(m^{2}\)=196then n=21 then n+6=27 non prime, for p=9q+2 then m=2(9q+2) for q=1 \(m^{2}\)=484 then n=53 then n+6=59 prime then n=53.

## Other useful links

- https://www.cheenta.com/smallest-perimeter-of-triangle-aime-2015-question-11/
- https://www.youtube.com/watch?v=ST58GTF95t4&t=140s

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