The refractive index of a prism with apex angle A is cot A2. The angle of minimum deviation is:
(4)
μ=sin(A+δm2)sinA2
cosA2sinA2=sin(A+δm2)sinA2
sin(π2-A2)=sin(A+δm2)
π2-A2=A2+δm2
δm=π-2A