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Try this beautiful problem from the PRMO, 2018 based on Smallest value.

## Smallest Value – PRMO 2018

Let a and b natural numbers such that 2a-b, a-2b and a+b are all distinct squares. What is the smallest possible value of b?

- is 107
- is 21
- is 840
- cannot be determined from the given information

**Key Concepts**

Algebra

Numbers

Multiples

## Check the Answer

But try the problem first…

Answer: is 21.

Source

Suggested Reading

PRMO, 2018, Question 15

Higher Algebra by Hall and Knight

## Try with Hints

First hint

2a-b=\(k_1^2\) is equation 1

a-2b=\(k_2^2\) is equation 2

a+b=\(k_3^2\) is equation 3

Second Hint

adding 2 and 3 we get

2a-b=\(k_2^2+k_3^2\)

or, \(k_2^2+k_3^2\)=\(k_1^2\) \((k_2<k_3)\)

Final Step

For least ‘b’ difference of \(k_3^2\) and \(k_2^2\) is also least and must be multiple of 3

or, \(k_2^2\)=a-2b=\(9^2\) and \(k_3^2\)=a+b=\(12^2\)

or, \(k_3^2-k_2^2\)=3b=144-81=63

or, b=21

or, least b is 21.

## Other useful links

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

Google