本题目来源于试卷: Geometry & Shapes，类别为 IB数学
[问答题]
In a triangle $\mathrm{ABC}, \mathrm{BA} \mathrm{A} C=60^{\circ}, \mathrm{AB}=(1-x) \mathrm{cm}$, $\mathrm{AC}=(x+3)^{2} \mathrm{~cm}$,$-3\lt x\lt1$
1. Show that the area, A $\mathrm{~cm}^{2}$ , of the triangle is given by
A=$\frac{\sqrt{3}}{4}\left(9-3 x-5 x^{2}-x^{3}\right)$ .
2. 2a Calculate $ \frac{\mathrm{d} A}{\mathrm{~d} x} $.
2b Verify that $\frac{\mathrm{d} A}{\mathrm{~d} x}=0 $ when $x=-\frac{1}{3}$ .
3. 3a Find $\frac{\mathrm{d}^{2} A}{\mathrm{~d} x^{2}} $ and hence verify that $x=-\frac{1}{3}$ gives the maximum area of triangle A B C .
3b Calculate the maximum area of triangle A B C .
3c Find the length of [B C] when the area of triangle A B C is a maximum.
[/B C]
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