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number theory math challenge II-B 3.28

 
 
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number theory math challenge II-B 3.28
by Yizhou Zhang - Tuesday, 25 May 2021, 11:31 PM
 

Hi, I am having trouble with this problem. I only got the answers 1 and 9 by listing the first few terms out. Can I have a hint of how to find every one of them instead of just the two?