Expert: David Montiel - 10/28/2009

Question
I am trying to check my son's homework but I'm stuck.It says to find the value of x so the function has the given value. An example is n(x)=7x+4;39. I know how to find x in 7x+4=39, xwould be 5. What's confusing me is what's the n for?

Answer
Hello Jennifer,

If I understand correctly, the question is: Find the value of x such that n(x)=39 given that n(x)=7x+4. Is that right? Now if that is the case what you're doing is correct, you are solving for x in the equation

7x+4=39

Don't be confused about the "n", it is just the name of the dependent variable (also referred to as the name of the function).

A function is usually expressed as an equation involving TWO variables: the independent variable ("x" in your example) and the dependent variable ("n" in the example). What's important is that in all functions, the dependent variable must be all by itself in one side of the "=" sign. So in the example

n=7x+4

you say that "n" is a function of "x".

The idea of a function is that as soon as you specify the value of the independent variable, you can calculate the value of the dependent variable. So, for instance, if you specify the value of "x" (say x=2) the value of the other variable ("n") gets fixed immediately,

n = 7*2+4
n = 18

Right?

In your example, you do the opposite: you fix the value n=39 and that determines the value of "x" (x=5).

Also please note that you can write "n" or "n(x)", they are the same thing. The "x" in parentheses is sometimes used to make it clear that we are referring to the dependent variable.

Hope this helps. Let me know if there is still something that is not clear.

Cheers,
David