Section 1-5 "Variables and Equations"

Technology objective: Students will use the TI-81 calculator to "test" whether a given value will make an equation true or false.

Lesson: Students should already know how to evaluate an expression for given variables to determine whether an equation is true or false. After this lesson, students can be shown how to use the "test" menu to test if the same equation is true or false.

Example 1) Which of the following numbers, 15, 17, or 19, is the solution for:

166 + b = 183

Students should be able to mentally compute which of the numbers is the solution for the equation. However, it is important to stress that we can use the calculators to determine whether a number will make the equation true or false.

TI-81 information

The calculator can test to see if an equation is true or false. Therefore, we will test all of the numbers by entering the following information. Remember, the calculator is not able to tell you "true" or "false." It does tell you by using "1" for true and "0" for false. Remember computers use binary code - a combination of 0's and 1's.

First enter the equation you would like to check. We will want to check if

166 + 15 = 183.

To test an equation to see if it is true or false:

1. Enter 166 + 15

2. Now look for the TEST menu, it is in blue above the MATH menu. To access this menu you will have to press the 2ND key then the MATH key.

3. You should see various symbols. We want to use the "=" symbol which it should be on. Therefore, press enter. You should see the "=" symbol on the screen.

4. Finally enter 183.

5. When you press enter, pay close attention to whether you get a "1" or a "0." Remember "1" means true and "0" means false.

Have students repeat those steps with the numbers 17 and 19. Which numbers came up false, which came up true? What does that mean?


Have students try the following to see which of the numbers makes the equation true and which of the following numbers makes the equation false.

Try b = 3, 4, and 5 for the equation

27 = 5b + 2

Also have them enter some equations that are obviously true and obviously false. (ex. 2 + 2 =4, 2 + 2 = 3)


Have the students repeat the steps for testing an equation in their journal. Also, have them discuss briefly, in a short paragraph, how the "test" function could be of use in the future.