I need help with geometry Let $ABCD$ be a convex quadrilateral. Let $|\measuredangle ABC|$=$|\measuredangle ACD|$ and $|\measuredangle ACB|$=$|\measuredangle ADC|$. Let $O$ be the circumcenter of triangle $BCD$, distinct from $A$. Prove, that $|\measuredangle OAC|$ is a right angle.
I'm supposed to use spiral similarity with center A and midpoints of sides $BC$ and $CD$ (let them be $M_A$ and $M_B$, respectively) to show that $AOM_ACM_B$ is a cyclic pentagon, but i don't know how to proceed with it. How do I do this?
 A: Particularly in the absence of better ideas, I like coordinates. Without loss of generality (as the setup is invariant under similarity transformations), you can use $A$ as the origin and $\overrightarrow{AB}$ as first unit vector. So you get
$$A=\begin{pmatrix}0\\0\end{pmatrix}\qquad
B=\begin{pmatrix}1\\0\end{pmatrix}\qquad
C=\begin{pmatrix}x\\y\end{pmatrix}$$
for some $x,y\in\mathbb R$. This defines the spiral similarity transformations, and applying that similarity to $C$ you get
$$D=\begin{pmatrix}x&-y\\y&x\end{pmatrix}\cdot
\begin{pmatrix}x\\y\end{pmatrix}=
\begin{pmatrix}x^2-y^2\\2xy\end{pmatrix}\;.$$
The midpoints are thus
$$M_A=\tfrac12(B+C)=\frac12\begin{pmatrix}x+1\\y\end{pmatrix}\qquad
M_B=\tfrac12(C+D)=\frac12\begin{pmatrix}x+x^2-y^2\\y+2xy\end{pmatrix}$$
and intersecting the orthogonal lines in these points you find that the circumcenter is
$$O=\frac{x^2+y^2-1}{2y}\begin{pmatrix}-y\\x\end{pmatrix}\;.$$
To show that $\overrightarrow{AC}$ and $\overrightarrow{AO}$ are orthogonal, you check that the scalar product between these two is zero:
$$\langle C,O\rangle=\frac{x^2+y^2-1}{2y}\left\langle
\begin{pmatrix}x\\y\end{pmatrix},
\begin{pmatrix}-y\\x\end{pmatrix}
\right\rangle=0\;.$$
