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/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.