1. Types of functions: Let's consider the following cell phone plans Unlimited minutes plan –$199 per month –No extra charge for minutes Pay by the minute plan –$19 per month –$0.20 per minute

2. linear function Recall slope of a line : rise = 120 run = 600 constant function The slope of the linear function is 120/600 = 0.2. This is reflected in the fact that the charge is $0.20 per minute. The slope of a constant function is always zero. or

3. Cost of monthly water consumption Instead of slope, we define "average rate of change" between two given x-values: Average rate of change from 10,000 to 15,000 gallons: This is a non-linear function. $3.50 per thousand gallons

4. The preceding function comes from this table The charge per 1000 gallons can be interpreted as the average rate of change. Note that for a non-linear function this number depends on your choice of x-values.