$\begin{array}{1 1} (a)\;\mu= \sin \bigg(\large\frac{A+D_m/2}{A/2}\bigg)\\ (b)\;\mu= \bigg(\large\frac{D_m/2}{A/2}\bigg) \\ (c)\;\mu= \large\frac{\sin A+D_m/2}{\sin A/2} \\ (d)\;\mu= \sin \large\frac{\sin(A+D_m)}{\sin(A)} \end{array} $
