[填空题]
In a triangle $ \mathrm{ABC}, \mathrm{AB}=2 \mathrm{~cm}, \mathrm{CBA}=\frac{\pi}{4}$ and$ \mathrm{BA} \hat{\mathrm{A}} \mathrm{C}=\theta$.
1. Show that $\mathrm{AC}=\frac{2}{\cos \theta+\sin \theta} $. AC =
2. Given that $\mathrm{AC} $ has a minimum value, find the value of $ \theta $ for which this occurs.
