The reason for the use of the theorem is the statement of the theorem itself, which has been proven in the steps above.

The reason for the use of the theorem is the statement of the theorem itself, which has been proven in the steps above.

The theorem states:
The angle subtended by an arc at the centre of a circle is twice the angle subtended by the same arc at any point on the remaining part of the circumference.

This is commonly known as the Angle at the Centre Theorem.

Reason for use of theorem:
The angle subtended by an arc at the centre is twice the angle subtended by the same arc at any point on the remaining part of the circumference.