How do you evaluate variable expressions?

1 Answer
Apr 26, 2015

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 : 3x-2y

In order to evaluate this expression, we'll need values (numbers) for both x and y.

Evaluate the expression 3x-2y 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