[填空题]
Let $f(x)=16-x^{2}$ , for $x \in \mathbb{R}$ .
1. Find the x -intercepts of the graph of f . x = $\pm $
The following diagram shows part of the graph of f .
Rectangle ABCD is drawn with $\mathrm{A} \& \mathrm{~B}$ on the x -axis and $\mathrm{C} \& \mathrm{D} $ on the graph of f . Let $\mathrm{OA}=a$ .
2. Show that the area of ABCD is $32 a-2 $a^{3}$$ . = 0
3. Hence find the value of a>0 such that the area of ABCD is a maximum.
Let $g(x)=(x-4)^{2}+k$ , for $x \in \mathbb{R}$ , where k is a constant.
4. Show that when the graphs of f and g intersect, 2 x^{2}-8 x+k=0 .
5. Given that the graphs of f and g intersect only once, find the value of k .
