A company expects to sell $D$ units of a certain product per year. Sales are assumed to be at a steady rate with no shortages allowed. Each time an order for the product is placed, an ordering cost of $K$ dollars is incurred. Each item costs $p$ dollars, and the holding cost is $h$ dollars per item per year.
a. Show that the inventory cost (the combined ordering cost, purchasing cost, and holding cost) is $$C(x)=\frac{K D}{x}+p D+\frac{h x}{2} \quad(x>0)$$
where $x$ is the order quantity (the number of items in each order).
b. Use the result of part (a) to show that the inventory cost is minimized if
$$x=\sqrt{\frac{2 K D}{h}}$$