Sunday, 4 June 2017

If n is any positive integer, prove that sqrt4n2 is irrational



If n is any positive integer, prove that 4n2 is irrational.



I've tried proving by contradiction but I'm stuck, here is my work so far:



Suppose that 4n2 is rational. Then we have 4n2 = pq, where p,qZ and q0.




From 4n2 = pq, I just rearrange it to:



n=p2+2q24q2. I'm having troubles from here, n is obviously positive but I need to prove that it isn't an integer.



Any corrections, advice on my progress and what I should do next?


Answer



The number 4n2 is rational iff 4n2=a2 reduction mod 4 shows that this is impossible.



Here is a proof of the general fact that k is irrational unless k is a square: Suppose uv is a solution to x2k=0, then it is an integer i by Gauss lemma, but then k=i2.


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