<s>[INST] Related to angles in tetrahedron Let $OABC$ be a tetrahedron such that $|OA|=|OB|=|OC|$. Denote by $D$ and $E$ the midpoints of segments $AB$ and $AC$ respectively. If $\alpha=\angle(DOE)$ and $\beta=\angle(BOC)$ what is the ratio $\beta/\alpha$? 
It is obvious that $|BC|=2|DE|$ since triangles $\Delta(ABC)$ and $\Delta(ADE)$ are similar and $D, E$ are midpoints by assumption. I would expect that $\beta/\alpha\geqslant 2$ but I haven't been able to confirm this. I tried to use the cosine law for the segments $|BC|$ and $|DE|$ but without success.  
 [/INST] This answer attempts to find a formula for the ratio.
I imagine this becomes finding the distance of two mid-points of a spherical triangle.
Construct a sphere centred at $O$ with radius $r=OA$.
Extend $OD$ and $OE$ to meet the sphere at $D'$ and $E'$ respectively; $D'$ and $E'$ will be the mid-points of arcs $\overset{\frown}{AB}$ and $\overset{\frown}{AC}$ respectively.
Following this convention of spherical trigonometry, consider the spherical triangle $ABC$, denote
$$\begin{align*}
a &= \angle BOC = \beta\\
b &= \angle AOC\\
c &= \angle AOB
\end{align*}$$
According to the spherical laws of cosine, if $A$ is the spherical angle made by arcs $\overset{\frown}{AB}$ and $\overset{\frown}{AC}$, consider spherical triangle $ABC$,
$$\begin{align*}
\cos A &= \frac{\cos a -\cos b \cos c}{\sin b \sin c}\\
&= \frac{\cos \beta -\cos b \cos c}{\sqrt{(1+\cos b)(1-\cos b)(1+\cos c)(1-\cos c)}}\tag 1\\
\end{align*}$$
And consider spherical triangle $AD'E'$,
$$\begin{align*}
\cos A  &= \frac{\cos \angle D'OE' -\cos \frac b2 \cos \frac c2}{\sin\frac b2 \sin\frac c2}\\
&= \frac{\cos \alpha -\sqrt{\frac{1+\cos b}2} \sqrt{\frac{1+\cos c}2}}{\sqrt{\frac{1-\cos b}2}\sqrt{\frac{1-\cos c}2}}\\
&= \frac{2\cos \alpha - \sqrt{(1+\cos b)(1+\cos c)}}{\sqrt{(1-\cos b)(1-\cos c)}}\tag 2
\end{align*}$$
Equating $\cos A$ in $(1)$ and $(2)$,
$$\begin{align*}
\frac{\cos \beta -\cos b \cos c}{\sqrt{(1+\cos b)(1+\cos c)}} &= 2\cos \alpha - \sqrt{(1+\cos b)(1+\cos c)}\\
\cos \beta -\cos b \cos c &= 2\cos \alpha\sqrt{(1+\cos b)(1+\cos c)} - (1+\cos b)(1+\cos c)\\
\cos \beta &= 2\cos \alpha\sqrt{(1+\cos b)(1+\cos c)} - 1 - \cos b - \cos c\\
\cos \alpha &= \frac{1 + \cos b + \cos c + \cos \beta}{2\sqrt{(1+\cos b)(1+\cos c)}}\\
&= \frac{1 + \cos b + \cos c + \cos \beta}{4\cos\frac b2 \cos \frac c2}\\
\frac \beta\alpha &= \frac{\beta}{\arccos \cfrac{1 + \cos b + \cos c + \cos \beta}{4\cos\frac b2 \cos \frac c2}}
\end{align*}$$
</s>