[填空题]
1. Expand and simpli 1n*ib/:c0bttrod8 hqfy+2f a:r0hyc5f1dmgj4 iq zof, $(1+\sqrt{3})^{2} $.
2. By writing $75^{\circ} $ as $30^{\circ}+45^{\circ}$ , find the value of $\cos 75^{\circ} $.
The following diagram shows the triangle $\mathrm{ABC}$ where $\mathrm{BC}=\sqrt{6}$, $\mathrm{CA}=2 $ and $ \mathrm{A} \hat{\mathrm{C} B}=75^{\circ}$ .
$\text { 3. Find } \mathrm{AB} \text { in the form } a+\sqrt{b} \text { where } a, b \in \mathbb{Z} \text {. }$
