Circle geometry · demo

Angle at the centre = 2 × angle at the circumference

Drag A, B and C around the circle. The angle AOB at the centre always stays twice the angle ACB, as long as A, B and C subtend the same arc.

shared arc AB centre ∠AOB circumference ∠ACB

Drag A, B or C around the circle

Measured angles

Show working
the centre-angle theorem, stated with these numbers

Working