a b c d

d

$ \mu = \frac{sin(\frac{A+\delta_{min}}{2})}{sin\frac{A}{2}} = cot \frac{A}{2}$ $ \Rightarrow sin(\frac{A+\delta_{min}}{2}) = cos\frac{A}{2} = sin(90 -\frac{A}{2}) $ $ \frac{A+\delta_{min}}{2} = \frac{180 -A }{2}$ $\Rightarrow \delta_{min} = 180 -2A$