The angle at the centre of a circle is twice the angle at the circumference when both are subtended by the same arc.

Proof: an arc BC is drawn on the circumference of a circle. Draw a line from these points to the centre of the circle (lines BO and CO). Draw also a line from these points to a point on the circumference (lines BA and CA).

A line from point A is extended through the centre to point D to create two triangles.

Angle ∠BAO = angle ∠ABO = `x` (AO, BO and CO are radii; both triangles are isosceles).

The value of angle ∠BOD = 2`x` (∠AOB = 180 - 2x; and ∠BOD = 180 - (180 - ∠AOB))

Similarly for the other triangle, ∠COD = 2`y`

Angle ∠BAC = `x` + y

Angle ∠BOC = 2`x` + 2`y` = 2(`x` + `y`)

Therefore the angle at the centre of a circle is twice the angles at the circumference when subtended by the same arc.