If n is any positive integer, prove that √4n−2 is irrational.
I've tried proving by contradiction but I'm stuck, here is my work so far:
Suppose that √4n−2 is rational. Then we have √4n−2 = pq, where p,q∈Z and q≠0.
From √4n−2 = 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 √4n−2 is rational iff 4n−2=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 x2−k=0, then it is an integer i by Gauss lemma, but then k=i2.
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