Check boxes for the following:
1. Show tangents from point C to circle,
2. Show line segments OC
Red dots for :
1. Moving the external point, C.

Pre-requisite knowledge and terms: Pythagoras Theorem, Perpendicular bisector, Radius, Chords, Tangents
Task: How do you prove that the tangents from the external points are equal in length?
(Hint: You may want to select "Show tangents from point C to circle". Besides the tangents, what are the dotted lines? What do they have in common? Select the check box "Show line segment OC" and recall the relationship between the sides in a pythagoras theorem. What is common between the 2 triangles, triangles OCD and OCE? Apply Pythagoras Theorem.)