Question 1151692
<pre>
When x=0 years have passed the value is y=$7200.
When x=9 years have passed the value is y=$3600.

So we want the equation of a line that goes through the points
(0,7200) and (9,3600)

{{{m=(y[2]-y[1])/(x[2]-x[1])}}}

{{{m=(3600-7200)/(9-0)}}}

{{{m=(-3600)/9}}}

{{{m=-400}}}

{{{y-y[1]=m(x-x[1])}}}

{{{y-7200=-400(x-0)}}}

{{{y-7200=-400x}}}

{{{y=-400x+7200}}}

or

{{{f(x)=-400x+7200}}}

The smallest value of x in the domain is when the largest value y is the full 
value of $7200.  That's when x=0 years.

The least value of x in the domain is when the smallest value is $0, when it is 
worthless.  We have to calculate that by substituting 0 for y and solving for x.

{{{0=-400x+7200}}}
{{{-400x=-7200}}}
{{{(-400x)/(-400)=(-7200)/(-400)}}}
{{{x=18}}}

So the domain is [0,18]  (years)

The range is [0,7200]  (dollars)

Edwin</pre>