problem
Orbiting billiard balls
What angle is needed for a ball to do a circuit of the billiard table and then pass through its original position?
Can you make a right-angled triangle on this peg-board by joining up three points round the edge?
What is the relationship between the angle at the centre and the angles at the circumference, for angles which stand on the same arc? Can you prove it?
Can you find triangles on a 9-point circle? Can you work out their angles?