How to Find the Value of a Function on the TI-84 Plus

By Jeff McCalla, C. C. Edwards

If you want to substitute a value in a function, you could accomplish this task by using paper and a pencil. However, wouldn’t it be easier to use your on your TI-84 Plus calculator to find the value of a function? There are a few different ways to accomplish this task.

The TI-84 Plus C displays functions and information in the border of the graph screen. The TI-84 Plus displays similar information directly on the graph screen.

Using your graph to find the value of a function

The CALC menu can be used to evaluate a function at any specified x-value. To access and use this command, perform the following steps:

  1. Graph the functions in a viewing window that contains the specified value of x.

    To get a viewing window containing the specified value of x, that value must be between Xmin and Xmax.

  2. Press [2nd][TRACE] to access the Calculate menu.

  3. Press [ENTER] to select the value option.

  4. Enter the specified value of x.

    When using the value command to evaluate a function at a specified value of x, that value must be an x-value that appears on the x-axis of the displayed graph — that is, it must be between Xmin and Xmax. If it isn’t, you get an error message.

    Use the keypad to enter the value of x (as illustrated in the first screen). If you make a mistake when entering your number, press [CLEAR] and re-enter the number.

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  5. Press [ENTER].

    After you press [ENTER], the first highlighted function in the Y= editor appears in the border at the top of the screen, the cursor appears on the graph of that function at the specified value of x, and the coordinates of the cursor appear in the border at the bottom of the screen. See the second screen.

    You can also find the value of a function by pressing [TRACE], entering an x-value, and pressing [ENTER].

  6. Repeatedly press the

    image1.jpg

    keys to see the value of the other graphed functions at your specified value of x.

    Each time you press the up- and down-arrow keys, the name of the function being evaluated appears in the border at the top of the screen and the coordinates of the cursor location appears in the border at the bottom of the screen. This is illustrated in the third screen.

After using the value command to evaluate your functions at one value of x, you can evaluate your functions at another value of x by keying in the new value and pressing [ENTER]. Pressing any function key (such as [ENTER] or [TRACE]) after evaluating a function deactivates the value command.

If you plan to evaluate functions at several specified values of x, consider constructing a user-defined table of functional values.

Using your calculator to find the value of a function

Another way to find the value of a function involves using your calculator. This method is easy and doesn’t have the restrictions the graphing method has (the x-value has to be between the Xmin and Xmax).

Follow these steps to use your calculator to find the value of a function:

  1. Enter your function in the Y= editor.

    You need to remember the name of the function you enter. An equation is entered in Y1 as shown in the first screen.

    image2.jpg

  2. Press [2nd][MODE] to access the Home screen.

  3. Press [ALPHA][TRACE] to access the Y-VAR menu and choose the function you need.

    See the second screen.

  4. Press [(] and enter the x-value you would like evaluated.

  5. Press [)] and then press [Enter].

    See the third screen.