I'm trying to find a reference to the following statement.
Define a function $f$ from the hyperbolic plane (in the Poincaré unit disc model using polar coordinates) to the Euclidean plane (using polar coordinates) by sending the point at hyperbolic distance $R$ from the origin at an angle $\theta$ to the positive $y$-axis to the point at Euclidean distance $R$ from the origin at an angle $\theta$ to the positive $y$-axis.
The rationale for why this should be $1$-Lipschitz is that the circumference of a circle of radius $R$ is $2\pi\sinh(R)$ in the hyperbolic plane and $2\pi R$ in the Euclidean plane, and $2\pi\sinh(R)\geq 2\pi R$ for all $R\geq 0$.
Unpacking the definitions of the metrics requires one to prove that \begin{equation} \cosh^{-1}\left(\cosh(r_1)\cosh(r_2)-\sinh(r_1)\sinh(r_2)\cos(\theta)\right) \geq \sqrt{r_1^2+r_2^2-2r_1r_2\cos(\theta)}. \end{equation} for all $r_1,r_2\geq 0$ and $\theta \in (-\pi,\pi]$, so I'm hoping there's a nicer way!
