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BdMO 2021 National Camp Discussion Thread

Posted: Fri Apr 23, 2021 4:27 pm
by Pro_GRMR
Hey, Campers!

This thread's purpose is:-

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1. To post our solutions to the problem sets. (Using Latex is easier this way)
2. To check each other's solutions and learn from them.
3. To share hints about the problems.
Good Practices to follow:-

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1. Use [hide][/hide] to hide your hint/solution.
2. Describe what you're posting.

Example

Posted: Fri Apr 23, 2021 4:43 pm
by Pro_GRMR
This is an example.
I'm sharing a hint for problem 1 of IMO Prep Pset-1.

Problem-
Find, with proof, all positive integers $n$ for which $2^n+12^n+2011^n$ is a perfect square.
Hint-
Did you know that all odd squares are $1$ modulo 8?

Re: Example

Posted: Fri Apr 23, 2021 4:53 pm
by MrCriminal
Pro_GRMR wrote:
Fri Apr 23, 2021 4:43 pm
This is an example.
I'm sharing a hint for problem 1 of IMO Prep Pset-1.

Problem-
Find, with proof, all positive integers $n$ for which $2^n+12^n+2011^n$ is a perfect square.
Hint-
Did you know that all odd squares are $1$ modulo 8?
Finding the magic number is tough for newbies like me ):

Re: Example

Posted: Fri Apr 23, 2021 5:17 pm
by Dustan
Pro_GRMR wrote:
Fri Apr 23, 2021 4:43 pm
This is an example.
I'm sharing a hint for problem 1 of IMO Prep Pset-1.

Problem-
Find, with proof, all positive integers $n$ for which $2^n+12^n+2011^n$ is a perfect square.
Hint-
Did you know that all odd squares are $1$ modulo 8?

Re: Example

Posted: Fri Apr 23, 2021 6:59 pm
by Pro_GRMR
Dustan wrote:
Fri Apr 23, 2021 5:17 pm
MrCriminal wrote:
Fri Apr 23, 2021 4:53 pm
Finding the magic number is tough for newbies like me ):
This is how I did it:
We see that $1$ obviously works.
Then show that the perfect square must be odd and so, $n$ will have to be even, If $n \geq 2$. If not then $2^n+12^n+2011^n\not\equiv1$(mod $8$)
And then we do the same thing as Dustan did in Case 1. And so, there exists only one such $n$ and it is $1$

Re: BdMO 2021 National Camp Discussion Thread

Posted: Fri Apr 23, 2021 9:21 pm
by Anindya Biswas
When $n>1$
First take mod $4$. It gives us $n$ even
Then consider mod $3$. It gives us $n$ odd
The rest should be clear.

Re: BdMO 2021 National Camp Discussion Thread

Posted: Sun Apr 25, 2021 10:44 am
by Asif Hossain
Next problem Pls :)