In a principal ideal domain, gcd(a,b) always exists and can always be expressed as $xa +yb$ with some $x, y /in R$

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In a principal ideal domain, gcd(a,b) always exists and can always be expressed as $xa +yb$ with some $x, y in R$.

I am quite new to algebra and would be really grateful if someone could show me ow to prove this.

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Consider the ideal $I := \{ xa + yb \mid x, y \in R \}$ (why is this an ideal?).

Since it is principal, it is generated by some element, say $r \in I$.

Show that $r$ is a gcd of $a$ and $b$.