Sol. $$ \begin{aligned} & \mu=\frac{\sin \left(\frac{A+\delta m}{2}\right)}{\sin \frac{A}{2}} \\ & \frac{\cos \frac{A}{2}}{\sin \frac{A}{2}}=\frac{\sin \left(\frac{A+\delta m}{2}\right)}{\sin \frac{A}{2}} \\ & \sin \left(\frac{\pi}{2}-\frac{A}{2}\right)=\sin \left(\frac{A+\delta_m}{2}\right) \\ & \frac{\pi}{2}-\frac{\mathrm{A}}{2}=\frac{\mathrm{A}}{2}+\frac{\delta \mathrm{m}}{2} \\ & \delta_{\mathrm{m}}=\pi-2 \mathrm{~A} \end{aligned} $$