A translation and a negation in $\mathbb{C}$ generate the infinite dihedral group. I'd like to show that the linear functions
$$ \varphi(z) = z+b, \;\;\; 0\neq b\in \mathbb{C}$$
$$ \psi(z) = -z+c, \;\;\; c\in \mathbb{C}$$
generate, under composition, a group isomorphic to $Dih_\infty$, the infinite dihedral group.
Now $Dih_\infty$ may be presented as follows:
$$ \langle r,s \, | \, s^2=1, srs=r^{-1}\rangle.$$
These relations are satisfied by $\varphi$ and $\psi$, since $\psi^2 = 1$, ($1$ stands for the identity function) and $\psi\circ \varphi \circ \psi = \varphi^{-1}$. How do I show no further relations hold ?
An alternative proof is also welcome.
 A: Without loss of generality, you can assume that $b=1$ and $c=0$.
Let $G$ be the group generated by $\phi$ and $\psi$. You know there is a surjection $f:D_\infty\to G$. Using the relations defining $D_\infty$ one can check easily that every element $g$ of $D_\infty$ is of the form $s^ar^b$ with $0\leq a<2$ and $b\in\mathbb Z$. 
If $f(g)$ is the identity, then $f(g)(\sqrt{-1})=\sqrt{-1}$. Show that in that case $a=b=0$.
A: An idea:
$$\tau(z):=\psi\circ\phi(z)=\psi(z+b)=-(z+b)+c=-z+(c-b)$$
$$\tau^2(z)=\tau(-z+(c-b))=-\left[-z+(c-b)\right]+(c-b)=z$$
And clearly,  $\;\phi=\psi^{-1}\tau=\psi\tau\;$ , thus our group is
$$\langle\;\psi\,,\,\tau\;;\;\psi^2=\tau^2=1\;\rangle=C_2*C_2\cong Dih_\infty$$
Of course, in order to be completely formal (or perhaps "too" formal) you may want to show that no finite product of the for $\,a_1b_1a_2b_2\cdot\ldots\cdot\;$ , with $\,a_i,b_i\in\{\psi\,,\,\tau\}\,\,,\,\,a_i\neq b_i\;$ , can be the identity map (trivial element) , but I think this is almost "trivial", since
$$\tau\circ\psi(z)=\tau(-z+c)=-(-z+c)+c-b=z-b\neq z\;,\;\;\text{since}\;\;b\neq 0$$
$$\text{and as already noted above,}\;\;\psi\circ\tau(z)=\psi(-z+c-b)=-(-z+c-b)+c=z+b\neq z$$ 
