Internal Group Direct Product Commutativity/General Result

Theorem
Let $\left({G, \circ}\right)$ be the internal group direct product of $H_1, H_2, \ldots, H_n$.

Let $h_i$ and $h_j$ be elements of $H_i$ and $H_j$ respectively, $i \ne j$.

Then $h_i \circ h_j = h_j \circ h_i$.

Proof
Let $g = h_i \circ h_j \circ h_i^{-1} \circ h_j^{-1}$.

From the Internal Direct Product Theorem, $H_i$ and $H_j$ are normal in $G$.

Hence $h_i \circ h_j \circ h_i^{-1} \in H_j$ and thus $g \in H_j$.

Similarly, $g \in H_i$ and thus $g \in H_i \cap H_j$.

But $H_i \cap H_j = \left\{{e}\right\}$ so $g = h_i \circ h_j \circ h_i^{-1} \circ h_j^{-1} = e$ and thus $h_i \circ h_j = h_j \circ h_i$.