Variable Expressions
Key Questions

Variable expressions are expressions that involve variables, which are symbols that represent changing quantities. (See http://socratic.org/questions/whatarevariables for reference). The value of the expression will change as the value of the variable changes.
For example, let's say with have the equation
#x + 5# When
#x = 1 # , then#x + 5 = 6# When
#x = 2 # then#x+5 = 7# Hope that was helpful.

To write variable expressions on Socratic, simply place hash tags around the expression when typing it:
[hash tag] x + 1 [hash tag] =
#x +1# [hash tag] (x1)/(x+1) [hash tag] =
#(x1)/(x+1)# Hope that was helpful.

To evaluate a variable expression means to find the value of the expression when the variable is (or variables are) replaced by a number (or numbers).
Here is a variable expression:
#3x+7# .To evaluate this expression when
#x = 2# mean to substitute#2# in place of#x# , and find the number (the value). [see note below]#3(2)+7 = 6+7 = 13# .When
#x=2# , the expression has a value of `#13# .If we evaluate the same expression when
#x=10# , we get:#3(10)+7 = 30+7 =37# .Here's a variable expression with two variables :
#3x2y# In order to evaluate this expression, we'll need values (numbers) for both
#x# and#y# .Evaluate the expression
#3x2y# when#x=4# and#y=5# #3(4)2(5) = 12  10 = 2# If we change the values to
#x=5# and#y=4# , we get
#3(5)2(4) = 15  8 =7# .Note:
To avoid saying exactly the same thing over and over, we use the following to mean the same thing:
evaluate the expression when
#x = 2#
evaluate the expression for#x = 2#
evaluate the expression at#x = 2#
evaluate the expression assuming#x = 2#
evaluate the expression if#x = 2# Also
If there is only one variable in an expression, we don't always need to mention it:
evaluate the expression for#2#
evaluate the expression at#2# 
Variables are symbols representing some changing quantities.
I hope that this was helpful.