Acute angle formed by a chord of a circle and a tangent to the circle at one of the endpoints of the chord.

### Example

In the figure below, angle *BAC* is a tangent-chord angle to the circle with centre *O*. The point of tangency is the endpoint *A* of chord *AB*. Line segment *AC* is a tangent to the circle at point *A*.

### Formula

In the example above, m\(\angle {\textrm{BAC}}=\frac{\textrm{m}\angle{\textrm{AOB}}}{2}\)