Use the difference formula of sines and evaluate the expression as, $\sin \left( 45{}^\circ -30{}^\circ \right)=\sin 45{}^\circ \cos 30{}^\circ -\cos 45{}^\circ \sin 30{}^\circ $ Substitute the values $\cos 45{}^\circ =\frac{1}{\sqrt{2}},\text{ }\cos 30{}^\circ =\frac{\sqrt{3}}{2},\text{ }\sin 45{}^\circ =\frac{1}{\sqrt{2}},\text{ and }\sin 30{}^\circ =\frac{1}{2}$. $\begin{align} & \sin \left( 45{}^\circ -30{}^\circ \right)=\left( \frac{1}{\sqrt{2}}\times \frac{\sqrt{3}}{2} \right)-\left( \frac{1}{\sqrt{2}}\times \frac{1}{2} \right) \\ & =\frac{\sqrt{3}}{2\sqrt{2}}-\frac{1}{2\sqrt{2}} \\ & =\frac{\sqrt{3}-1}{2\sqrt{2}} \end{align}$ Hence, the exact value of $\sin \left( 45{}^\circ -30{}^\circ \right)$ is equivalent to $\frac{\sqrt{3}-1}{2\sqrt{2}}$.