Find the Value of a Function on TI83 Plus
When you trace the graph of a function on the TI83 Plus graphing calculator, the trace cursor doesn’t hit every point on the graph. So tracing is not a reliable way of finding the value of a function at a specified value of the independent variable x. The CALC menu, however, contains a command that will evaluate a function at any specified xvalue.
To access and use this command, perform the following steps:

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.

Set the Format menu to ExprOn and CoordOn.

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

Press [1] to select the value option.

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 xvalue that appears on the xaxis 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. If you make a mistake when entering your number, press [CLEAR] and reenter the number.

Press [ENTER].
After you press [ENTER], the first highlighted function in the Y= editor appears 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 at the bottom of the screen.

Repeatedly press
to see the value of the other graphed functions at your specified value of x.
Each time you press
the name of the function being evaluated appears at the top of the screen and the coordinates of the cursor location appears at the bottom of the screen.