\(\mu = \frac{{\sin \,\left({\frac{{A + {\delta _m}}}{2}} \right)}}{{\sin \,\left({\frac{A}{2}} \right)}}\)

\( \Rightarrow \)\(\mu = \frac{{\sin \left({\frac{{A + A}}{2}} \right)}}{{\sin \left({\frac{A}{2}} \right)}} = 2\cos \frac{A}{2} \)

\(\Rightarrow A = 2{\cos ^{ - 1}}\left({\frac{\mu }{2}} \right)\)