Evaluating Expressions

Date: 08/12/97 at 17:34:43
From: Salsa Montgomery
Subject: Pre-algebra (variable expressions)

Dear Dr. Math,

I have tried to figure this problem out for a while. I read the 
instructions in the book over and over but I still can't figure this
problem out:

Evaluate the variable expression when x=4.  5x and 13-x

What I don't understand is how you evaluate the variable expression
and get the answer.

Thanks!

Date: 08/12/97 at 18:01:48
From: Doctor Mike
Subject: Re: Pre-algebra (variable expressions)

Hi!  Good question.  You have to know what it means to 
evaluate before you can do it.  

Basically, to evaluate means to find the *value* of the 
expression.  That means to find a number.
  
For your "5x" example, the expression means "whatever x is,
take 5 times it".   If somebody (teacher, book, parent, etc.)
tells you what x is, then that starts it off.  If x = 4 then
you KNOW what x is.  To get the value you do just exactly 
what the expression says.  If this case it is 5*x = 5*4 = 20.
   
For your "13-x" example, the expression means "whatever x is,
subtract it from 13".  If you do exactly what the expression
says you will get the value.  So let's do it.  You are given
again that x is 4.  What happens when you subtract 4 from 13?
Right, you get 9, so ....
    
     The value of the expression "13-x" is 9, if x is 4.
     You also say that "13-x" evaluated with x = 4 gives 9.
   
Some people talk about "plugging in" the value that is
given for x, and then getting the value for the expression.
   
I hope this helps get you started.  Write back if you have
another question.  Bye for now.   

-Doctor Mike,  The Math Forum
 Check out our web site!  http://mathforum.org/dr.math/