To facilitate the sought proof, we need to enhance the scenario described in OP’s problem statement as shown in the diagram. We have marked point $E$ on the side $BC$ such that $BE=BD$. We have introduced a circular arc with radius equal to $BC$, the center of which is at the vertex $B$, to cut extended $BA$ and $BD$ at $G$ and $H$. Finally, after joining $H$ to $C$ and $G$, a line is drawn through $D$ parallel to $HG$ to meat $BG$ at $F$.
The proof follows as soon as the missing angles marked with small arcs are found through angle chasing. We would like you to work out the proof yourselves using the hints we have given here. In case you find it still too difficult to do it, let us know and we will oblige with the complete solution.
$\underline{\text{Edit to Add the Proof Upon OP’s Request}}:$
$\angle GAC$ and $\angle CAB$ are supplementary angles, so we have,
$$\measuredangle GAC = 180^o - \measuredangle CAB =180^o - 100^0=80^o. \tag{1}$$
Since $BH$ and $BG$ are radii of the added arc, $\triangle GBH$ is an isosceles triangle. We have drawn $DF$ parallel to $HG$. Hence, $\triangle FBD$ is also an isosceles triangle and that makes $\measuredangle DFB=80^o$. we have already shown that $\measuredangle CAB=80^o$ (see (1)). As a consequence, $\triangle ADF$ is an isosceles triangle. Therefore, we have,
$$AD = FD. \tag{2}$$
Applying the intercept theorem to $\triangle GBH$ and the segment $FD$, we shall write,
$$\dfrac{FD}{GH}=\dfrac{BD}{BH}. \tag{3}$$
Because they are radii of the arc, the two sides of $\triangle HBC$, i.e., $BC$ and $BH$, are equal. Since we have selected $E$ on the side $BC$ such that $BE=BD$, segment $ED$ is parallel to $CH$. Therefore, we can apply the intercept theorem to $\triangle HBC$ and the segment $FD$ to obtain,
$$\dfrac{ED}{CH}=\dfrac{BD}{BH}. \tag{4}$$
The two chords $CH$ and $GH$ of the arc subtend equal angles at the center $B$ of the arc,
$${\large{\pmb{\therefore}}}\quad CH=GH. \tag{5}$$
From (2), (3), (4), and (5), we can deduce that,
$$ED=FD=AD. \tag{6}$$
$\triangle BED$ is another isosceles triangle, because $BD=BE$. Therefore, we know that,
$$\measuredangle BED=\dfrac{180^o-\measuredangle DBE}{2}=80^o.$$
Since $\angle BED$ is an external angle of $\triangle DEC$, we have,
$$\measuredangle CDE = \measuredangle BED - \measuredangle ECD =80^o-40^0=40^o.$$
Since $\measuredangle ECD$ of $\triangle DEC$ is also $40^o$, this triangle is an isosceles triangle as well.
$${\large{\pmb{\therefore}}}\quad CE=DE. \tag{7}$$
Using (6) and (7), we can show that $CE=AD$. Finally, we have,
$$AD+DB=CE+EB=CB.$$
Best Answer
Indeed you are proceeding correctly. The equation can be solved as follows: $$\sin 2\theta \sin \theta=\sin 4\theta(\sin 3\theta-\sin 2\theta) {\tag 1}$$ Now $\sin 3\theta-\sin 2\theta=2\cos \frac {5\theta}{2} \sin \frac {\theta}{2}$.
Also $\sin \theta=2\cos \frac {\theta}{2} \sin \frac {\theta}{2}$, and $\sin 4\theta=2\sin 2\theta \cos 2\theta$. So, $(1)$ simplifies to: $$\cos \frac {\theta}{2}=2\cos \frac {5\theta}{2}\cos 2\theta {\tag 2}$$ Since $2\cos \frac {5\theta}{2} \cos 2\theta=\cos \frac {9\theta}{2}+\cos \frac {\theta}{2}$, we have, from $(2)$: $$\cos \frac {9\theta}{2}=0$$ This means that $\frac {9\theta}{2}=\frac {\pi}{2}$, hence $\theta=\frac {\pi}{9}$. This means that all angles of triangle are known, and we know $BC=4$. Thus using sine law, $$2R=\frac {BC}{\sin A}$$ is easy to calculate.