Question

How do you evaluate variable expressions?

Answer 1

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`