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Writing a Function Rule

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This lesson is how to write a function rule. This video shows how functions typically behave, and how to write them down. Functions usually have a variable with a coefficient in front that represents the rate of change and has a constant value that represents the starting point of the function.
The setup for this would look like:

Where # represents a constant
 y = (\#) x + (\#)
( (#x)  \rightarrow This is the starting point, x = 0 )
( (#)  \rightarrow This is the constant change )

Video-Lesson Transcript

We’re going to cover how to write a function rule for particularly linear functions.

Linear function has a basic form:

y = \#x + \#

Where the \# in \#x represents the constant change

and \# is the starting point. The starting point is where x = 0.

For example:

We have a cellphone plan that requests \$25 at sign up and requires \$60 per month. What is the function of the cost?

It will be cost equals 25 start-up fee plus 60 multiplied by the months.

So we’ll have

c = 60m + 25

Writing a Function Rule

This relates very closely with the basic form of linear equation which is y = \#x + \#.

Let’s look at another example.

A bank account has \$200 in it. Every week, \$20 is added to it. Write a function to represent the amount of money (m) in the bank account after (w) weeks.

The money will be equal to starting amount plus the amount you add to it times the number of weeks that passed.

m = 200 + 20w

After 8 weeks, the amount of money you have is computed as:

m = 200 + 20(8)
m = 200 + 160
m = 360

Now, let’s see how much money you make after 20 weeks.

So, we’ll substitute 20 for w.

m = 200 + 20(20)
m = 200 + 400
m = 600

Here, our function can be rewritten as m = 20w + 200

which follows the basic linear equation form of y = \#x + \#.