Explain how industry sales and industry profits behave over the product life cycle.

The Product Life Cycle Diagram from the following reference shows how
sales and profits vary over the product life cycle.  They typically
resemble a bell curve, rising from 0 as the product is introduced,
growing during the growth phase, peaking during maturity, and then
declining as the product becomes obsolete or loses its novelty. 
Discontinued products return to 0, thereby completing the cycle.

Sincerely,

Wonko

"Product Life Cycle" QuickMBA.com (2004)
http://www.quickmba.com/marketing/product/lifecycle/