Let $R$ be a euclidean domain, with norm function $N: R^{*} \to N$.

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Show that $a, b \in R^*, N(a) = N(ab)$ if, and only if, $a$ and $b$ are associate.

I tried to solve this using the fact that if $N(a) = N(ab)$ then $a \in <ab>.$ But I failed. Any Help?