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# IMO past problem Watch

1. So I've been attempting to try and answer question A2 on the following: http://www.imo-official.org/problems/IMO2013SL.pdf

I haven't looked at the solution, so I'm not really sure where to start. The only thing I can think of so far is denoting a variable to be the distance between the numbers within the set, and then coming up with a formula such that the largest distance between the numbers is say x-y where x is the largest number and y the smallest.

Any help appreciated.
2. It's been a long time since I did anything like this but here's a first thought of one possible line you could take. Clearly if you can find a,b,c,d such that a-b =c-d then the problem is done. So you should assume that this condition is false and see what that means.

Spoiler:
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once you know what these sets look like the next step could be, for example, that you assume WLOG that a-b<c-d (why?) and remove the mod sign

A second first approach could be to ask yourself what the RHS means specifically.

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Updated: July 20, 2014
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