Normed Division Ring Completions are Isometric and Isomorphic/Lemma 1

Theorem
Let $\struct {R, \norm {\, \cdot \,} }, \struct {S_1, \norm {\, \cdot \,}_1 }$ and $\struct {S_2, \norm {\, \cdot \,}_2 }$ be normed division rings.

Let $\phi_1: R \to S_1, \phi_2: R \to S_2$ be distance-preserving ring monomorphisms. Let $\psi’ = \phi_2 \circ \phi_1^{-1}:\phi_1 \paren R \to \phi_2 \paren R$ be the composition of $\phi_1^{-1}$ with $\phi_2$.

Then $\psi': \struct {\phi_1 \paren R, \norm {\, \cdot \,}_1 } \to \struct {\phi_2 \paren R, \norm {\, \cdot \,}_2 }$ is an isometric ring isomorphism.

Proof
By Monomorphism Image Isomorphic to Domain then $\phi_1:R \to \phi_1 \paren R$ and $\phi_2:R \to \phi_2 \paren R$ are ring isomorphisms.

By Distance-Preserving Image Isometric to Domain for Metric Spaces then $\phi_1:R \to \phi_1 \paren R$ and $\phi_2:R \to \phi_2 \paren R$ are isometries.

By Inverse of Algebraic Structure Isomorphism is Isomorphism then $\phi_1^{-1}:\phi \paren R \to R$ is a ring isomorphism.

By Inverse of Isometry of Metric Spaces is Isometry then $\phi_1^{-1}:\phi \paren R \to R$ is a ring isomorphism is an isometry.

By Composition of Ring Isomorphisms is Ring Isomorphism then $\psi’$ is a ring isomorphism.

By Composition of Isometries is Isometry then $\psi’$ is an isometry.