Problem

甲、簡答題：共 8 題，每題 8 分，共 64 分。請在答案卷上列出題號依序作答。

4. A building in the shape of a rectangular box is to have a volume of $12{,}000\ \mathrm{ft}^3$. It is estimated that the annual heating and cooling costs will be \2$/square foot for the top,$$4$/square foot for the front and back, and$$3$/square foot for the sides. What is the minimal annual heating and cooling cost?