Chapter 3 - Derivatives - 3.6 Derivatives as Rates of Change - 3.6 Exercises - Page 182: 21

\[\begin{align} & \text{a}\text{. }\overline{C}\left( x \right)=\frac{1000}{x}+0.1,\text{ }C'\left( x \right)=0.1 \\ & \text{b}\text{.}\overline{C}\left( 2000 \right)=\$0.60/\text{item},\text{ }C'\left( 2000 \right)=\$0.10/\text{item } \\ & \text{c}\text{.The average cost per item when 2000 items are produced is } \\ & \$0.60/\text{item}\text{.} \\ & \text{The cost of producing 2001st item is 0}\text{.10. }\!\\\!\!\text{ } \\ & \\ \end{align}\]

\[\begin{align} & \text{Let }C\left( x \right)=1000+0.1x,\text{for the interval }0\le x\le 5000,\text{ }a=2000 \\ & \\ & \text{a}\text{. The average cost function is given by: }\overline{C}\left( x \right)=\frac{C\left( x \right)}{x},\text{ then} \\ & \overline{C}\left( x \right)=\frac{C\left( x \right)}{x}=\frac{1000+0.1x}{x} \\ & \text{Simplifying} \\ & \overline{C}\left( x \right)=\frac{1000}{x}+0.1 \\ & \text{The marginal cost function is }C'\left( x \right) \\ & C'\left( x \right)=\frac{d}{dx}\left[ 1000+0.1x \right] \\ & C'\left( x \right)=0.1 \\ & \\ & \text{b}\text{. The average and marginal cost when }x=a=2000\text{ is} \\ & \overline{C}\left( 2000 \right)=\frac{1000}{2000}+0.1 \\ & \overline{C}\left( 2000 \right)=0.6 \\ & or \\ & \overline{C}\left( 2000 \right)=\$0.60/\text{item} \\ & \\ & and \\ & \\ & C'\left( 2000 \right)=0.1 \\ & C'\left( 2000 \right)=\$0.10/\text{item} \\ & \\ & \text{c}\text{. From the result of the part b, the average cost per item when} \\ & \text{2000 items are produced is }\$0.60/\text{item}\text{.} \\ & \text{The cost of producing 2001st item is 0}\text{.10. }\!\\\!\!\text{ } \\ \end{align}\]