Friday, 2 June 2017

proof verification - Prove or disprove if n is an odd integer then n21 is divisible by 8

Hey I'm stuck on this question on how to prove it, I can prove if it's divisible by 4 but I'm unsure how to do it for 8.



Question




Prove or disprove the following:



If n is an odd integer then n21 is divisible by 8 (and 16 is the next question).



Note



I know they're true and false respectively but I'm not sure how to prove it, if you could show me how to do both that would be greatly appreciated!



Working




nZoddn=2k+1 for some kZ





= 4k^2+4k+1-1



=4k^2+4k



4(k^2+k)




Since 4|8 \implies 4(k^2+k)|8



\therefore (n^2-1)|8



QED



As you can see from the proof I can easily show that (n^2-1)|4 however I'm not sure what to do after it, as 4\nmid 8 (I'm pretty sure)



Thanks!

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