 Variables can be added and subtracted to make an expression simpler.  For example, consider the following expression:2x + 3x + 4x How many x's are in this expression? There are three terms in the expression. The first term contains two x's. The second contains three x's. The third has four x's. Altogether there are nine x's. 2x + 3x + 4x = 9x The variables are counted just like apples! Here's another example: 3a + a + 7a How many a's are in the expression? Just add them ... 3a + a + 7a = 11a Did you remember to count the 1a in the middle? When there's only one of a variable, the 1 doesn't have to be written. So a is the same as 1a. Here's another example, using subtraction as well: 12y - 8y + 2y How many y's are in the expression? Just combine them by subtracting and adding ... 12y - 8y + 2y = 6y Here are some more for you to try. If you place your mouse cursor in the box to the right of each question, you'll see the answer.
When you're evaluating an expression, it's sometimes easier to simplify it first before filling in the number. Here's an example so you can see what we mean: Evaluate 4x + 2x + x if x = 3 Here's how to do it: 4x + 2x + x = 7x <<< simplify it first by counting the x's = 7(3) <<< then fill in the number = 21 Here's one more example: Evaluate 5a - 3a + 4a if a = 5 5a - 3a + 4a = 6a = 6(5) = 30 Now let's look at how to guess the value of the variable when you know the answer. It's called solving the equation |
