The length of deviation for a glass prism is equal to its...

Here, $\delta_m=A, \mu=1.5$ \therefore \mu=\frac{\sin \frac{\left(A+\delta_m\right)}{2}}{\sin \frac{A}{2}}=\frac{\sin \frac{(A+A)}{2}}{\sin \frac{A}{2}} =\frac{2 \sin \left(\frac{A}{2}\right) \cos \left(\frac{A}{2}\right)}{\sin \left(\frac{A}{2}\right)} \frac{3}{2}=2 \cos \frac{A}{2} \quad$\left(\because \mu=1.5=\frac{3}{2}\right)$\cos \frac{A}{2}=\frac{3}{4} \frac{A}{2}=\cos ^{-1}\left(\frac{3}{4}\right)$or$A=2 \cos ^{-1}$\left(\frac{3}{4}\right)$