User-defined functions can be defined then used in various HP Prime applications or during Home or CAS operations. As usual, you can check out the online help before using this feature. In the examples below, we will also use the Function App to plot the user defined functions.
Example 1 - define a polynomial then plot in Function App
Say you wanted to define and plot an expression AX2 + BX + C where the coefficients A,B and C can be adjusted by you as needed. The following will define a new expression:
There is now a new expression polynom_ABC saved in Toolbox/User/User Functions. Note the order of the variables (X,A, B, C). This is important when using this definition later.
Now, open the Function App, Symb view. In the top function F1(X)= insert the new polynom_ABC function. Key in the coefficient values for (X,A, B, C) using X, 1, -1 and 1. Finally, you can use copy/paste to enter this function in F2(x) through F6(x).
Your Function App Symb screen should look similar to below. You can use your own values for A, B and C if you wish.
To plot these values, press the Plot key. The curves shown below are F1-F6. If you want to see how other A,B,C coefficients affect curve(s), simply return to the Symb view and edit the coefficients.
Try the following on your own:
 Depending on whether you started in Home or CAS mode, you may need to use Shift to enter lower case characters. I used lower and uppercase, but either will work. Keying this in on the emulator is easier.