Given a semicircle with $O$ as the centre, as shown in the figure, the ratio $\dfrac{\overline{AC}+\overline{CB}}{\overline{AB}}$ is ________ where $\overline{AC},\overline{CB}$ and $\overline{AB}$ are chords.

1. $\sqrt{2}$
2. $\sqrt{3}$
3. $2$
4. $3$

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