Prove that in ring $\mathbb{Z}, \forall a,b \in \mathbb{Z} $, $$(a,b) = (d),$$ where $ d = \mathrm{gcd} (a,b)$. Also, generalize this for all finite generated ideals in $\mathbb{Z}$.
I don't even know how to start. I am not sure if I got the definition of generated ideal. When I have $(a)$, what does that mean?