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