Testing point on circle
Drag the point C and observe its distance from A.
Discuss the meaning of the expressions and how they can be used to distinguish points on the circle.
You may use 
to attach point C to the circle or detach from it.
to attach point C to the circle or detach from it.The video below shows a similar activity with a simple drawing in GeoGebra and some dynamic calculation in the ipad Maths Note.
There is another version in Polypad. We can roll dice to generate numbers for users to choose as coordinates of points to test. It can be a game about who can make it close to the circle. The table keeps record of the points tested for further investigation and discussion.
Go to polypad.org/JSfSk5rWjPu9g