Circle theorem 5: angle at the centre

Topic:
Circle
Play with the points around the edge of the circle. What do you notice about the angles?
If the angle at the centre is 120°, what will the angle at the circumference be? If the angle at the circumference is 55°, what will the angle at the centre be? If the angle at the circumference is ϑ° how would you find the angle at the centre? How can you write this using algebra? (ϑ = "theta" is a Greek letter used by mathematicians to stand for an unknown or variable angle like we use x to stand for an unknown or variable number)