Right Triangle and Circle Theorem Let $ABC$ be a triangle such that $\angle BAC$ is a right angle. Suppose $D$ is a point lying on $BC$ such that $BD=1$, $DC =3$ and $\angle ADB=60^{\circ}$, find the length of $AC$.
I was told that there is a neat way to solve the question by considering the circumcircle of $\triangle ABC$. Since $\angle BAC=90^{\circ}$, $BC$ must be a diameter. Let $M$ be the midpoint point $BC$, then it's the centre of the circumcircle. But then I couldn't see anything substantial, please helps,

The circumcircle provides a good approach.  $|DM|=1$, $|AM|=2$, and $|MC|=2$.  Because $\angle ADB = 60^{\circ}$, $\angle ADM = 120^{\circ}$.  We may then find $\angle DAM$ using the law of sines:
$$\frac{\sin{\angle DAM}}{|DM|} = \frac{\sin{\angle ADM}}{|AM|} \implies \sin{\angle DAM} = \frac12 \sin{120^{\circ}} = \frac{\sqrt{3}}{4} $$ 
We may therefore find $\angle AMD = 180^{\circ} - \angle DAM - \angle ADM$ and therefore $\angle AMC = 180^{\circ} - \angle AMD =  \angle DAM + \angle ADM$.  We may now use the law of cosines to find $|AC|$:
$$\begin{align}|AC|^2 &= |AM|^2+|MC|^2 - 2 |AC| |AM| \cos{\angle AMC}\\&= 2^3 [1- \cos{(120^{\circ}+\angle DAM)}]\\&= 16 \sin^2{\left (60^{\circ}+\frac12 \angle DAM \right )}\\ &= 4 \left (\sin^2{\left (\frac12 \angle DAM \right )}+3 \sin^2{\left (\frac12 \angle DAM \right )} + \sqrt{3} \sin{\angle DAM}\right ) \\ &= 4 \left (\frac{4-\sqrt{13}}{8} + \frac{12+3 \sqrt{13}}{8} + \frac34 \right ) \\ &= 11+\sqrt{13} \end{align}$$
Thus

$$|AC| = \sqrt{11+\sqrt{13}} $$

ADDENDUM
This is even easier when one sees that $\angle ABC = 1/2 \angle AMC$.  Also, the previous result I had was correct but not simplified enough.
