In economic theory, we are frequently concerned with such things as trying to maximize profits or minimize costs.

Profits are at a maximum, for example, when the change in profits divided by the change in output is equal to zero.

Costs are at a minimum when the change in costs divided by the change in output is equal to zero. i.e. When the slope of the cost function is zero.

Usually, when we are interested in maximum or minimum values, we will be working with non-linear function.