Question

the number of people in the united states who used a cell phone in 2004 was 180 million, and 285 million in 2009. if the growth in cell phones usage was linear, what was the rate of change from 2004 to 2009?

Explanation:

Step1: Identify the two - point data

Let 2004 be \(x_1 = 0\) with \(y_1=180\) (in millions), and 2009 be \(x_2 = 5\) (since \(2009 - 2004=5\)) with \(y_2 = 285\) (in millions).

Step2: Use the slope formula

The rate of change (slope \(m\)) of a linear function is given by \(m=\frac{y_2 - y_1}{x_2 - x_1}\). Substitute \(x_1 = 0,y_1 = 180,x_2 = 5,y_2 = 285\) into the formula: \(m=\frac{285 - 180}{5-0}\).

Step3: Calculate the value

\(m=\frac{105}{5}=21\).

Answer:

21 million per year