Everything in Geogebra is a variable. The lists, the functions, the coordinates. Most things can be referenced and connected to other operations in a file. Below is an example of how a simple variable value can be referenced from a point on the screen.[br][br]In the example below, you can drag Point A and it will control point B. In the Algebra window, you can change the equation f(x) as well as see how B is defined. I think of it has function notation said out loud. x(A) in my mind is [b]the x value of Point A.[/b]

Sometimes you want to control a value using a slider instead of a point. It can limit the inputs, it can keep the controls in a place where can see time if you change an equation. Sliders can also control the steps of a construction. Below you can see how a number works, how an angle works (you need to use the angle for any angle or rotation) and integer when you don't want to use decimals in your values.

Sometimes, you'd like the applet to be a little cleaner without the algebra window being shared. A textbox would still allow for the control of variables and equations. Also, you can use a textbox as a way to check understanding with a "you got it" pop up. (Reverse engineer to see how to conditionally format any object--not just text)