When a line subtends an angle, lines drawn from its ends form that angle at the point where they meet

It can be proved that the angle subtended by an arc of a circle at the centre of a circle is twice the angle subtended by that arc at a point on the circumference of the circle. 可以证明，圆弧在圆心所切的角是该圆弧在圆周上某点所切角的两倍。