# Precalculus : Determine the Equation of a Linear Function

## Example Questions

### Example Question #51 : Graphing Functions

What is the equation of the line that passes through the points  and ?

Explanation:

First, we need to compute , the slope. We can do this with the slope formula

, sometimes called "rise over run"

So we now have

Now in order to solve for  we substitute one of our points into the equation we found. It doesn't matter which point we use, so we'll use .

We then have:

Which becomes .

Hence we take our found value for  and plug it back into  to get

### Example Question #2 : Determine The Equation Of A Linear Function

Find the equation of the line with slope  that passes through the point .

Explanation:

Since our slope is , we can plug it into  right away giving .

To solve for , we plug our given point  in for  and  giving .

This will simplify to , or  after subtracting the fraction.

Hence our answer is  after plugging our values for  and  in.

### Example Question #51 : Graphing Functions

Suppose Bob has 4 candies.  He then earns candies at a rate of 14 candies per week.  Which of the following formulas is the most reasonable rate to know how many candies Bob has on any given day?

Explanation:

Bob starts out with 4 candies.  Write the equation.

Every week, he earns 14 candies.  Every week has a total of 7 days.  Divide the amount of candies by the number of days to determine the number of candies Bob earn in a day.

Bob then earns 2 candies per day.  Let  represent the number of candies per day.  Finish the incomplete equation.

### Example Question #1 : Determine The Equation Of A Linear Function

After one hour of working, Billy has three dollars.  At the end of eight hours, Billy has fifty dollars.  Choose the most correct linear function representing this scenario if  represents time, and  represents dollars.

Explanation:

To solve for the equation of this problem, we will need 2 points.  Let the point be defined as , where  is time in hours and  represents the dollars Billy has earned.

Billy starts with three dollars in the first hour.  Write the point.

At the end of eight hours, Billy has fifty dollars.  Write the second point.

Write the slope-intercept formula.

Write slope formula.

Plug a point and the given slope to the slope-intercept formula to solve for the y-intercept.

Substitute this and the slope back into the slope-intercept equation.

Since Billy's earning is dependent on the time he works, rewrite the equation so that time is the independent variable, and amount earned is the dependent.

### Example Question #1 : Determine The Equation Of A Linear Function

Determine the equation of a line that passes through the points  and .

Explanation:

Linear functions follow the form , where m is the slope and b is the y intercept. We can determine the equation of a linear function when we have the slope and a y intercept which is a starting point for drawing our line. If we need the equation of a line that passes through two points we use the point slope equation:

where the variables with numbers next to them correspond to places where you input the x and y from a single (same) coordinate pair.

Since we have two points, we can compute the slope of a line that passes between them.

.

Now use the point slope formula:

distribute the right side

Take a look at the coordinate pairs. Was it necessary to to use the point slope formula? The answer is no, since the coordinate pair (0,5) is already a y intercept. So all that was necessary was to compute the slope.

### Example Question #51 : Graphing Functions

What equation is perpendicular to  and passes throgh ?

Explanation:

First find the reciprocal of the slope of the given function.

The perpendicular function is:

Now we must find the constant, , by using the given point that the perpendicular crosses.

solve for :

### Example Question #1131 : Pre Calculus

What equation is perpendicular to  and passes throgh ?

Explanation:

First find the reciprocal of the slope of the given function.

The perpendicular function is:

Now we must find the constant, , by using the given point that the perpendicular crosses.

solve for :

### Example Question #7 : Determine The Equation Of A Linear Function

What equation is perpendicular to and passes throgh ?

Explanation:

First find the reciprocal of the slope of the given function.

The perpendicular function is:

Now we must find the constant, , by using the given point that the perpendicular crosses.

solve for :