What does GeoGebra prove?
A tangent touches a circle at B and BD is a diameter of the circle. E and F are points on the tangent. ED intersects the circle at H and FD intersects the circle at J. Prove that EHJF lie on the circumference of a circle.
This was a problem in the textbook for my Year 11 class. Solve it, and do as we did, create a GeoGebra demonstration for the theorem.
If you are a maths teacher you may like to read this.