Define Functions and Graph Them in TINspire Graphs Application
The Calculator application and the Graphs application are a perfect fit. You can define functions in the TINspire Calculator application and graph them in the Graphs application.

Use the Define command to define area(r).
In the first screen, the circle area formula is used.

Press [CTRL][I] and open a new Graphs page.
By default, you’re in graphing mode with the cursor located on the entry line next to the first available function.

Press [VAR] to view a list of variables and select area from the list.
You can also type the variable name using the alpha keys. If you do, don’t forget to place an open parenthesis after the variable.

Type the variable x as the independent variable (even though you used r when defining the function).

Press [ENTER] to graph the function and adjust the window settings accordingly.
Some lines and a moveable point are included, with coordinates, on the graph. (See the third screen.)
Because you graphed the function area(r) in the Graphs application as f1(x), you can now access this function in two ways, area and f1. For example, from within a Calculator page, you can type area(4) to find the area of a circle with radius 4. Alternatively, you can evaluate f1(4) and obtain the same result.