Hello,

I assume that CE is a tangent to the circle.

If so

is an isosceles triangle with base

.

Let M be the centre of the circle and CE is tangent to the circle:

1.

2.

3.

4. (CM) is perpendicular bisector of (AB) thus

is an isosceles triangle.

5. thus

6.

is an exterior angle.

7.

8. Therefore:

9. Therefore

10.The arc(BC) belongs to the angle

. You only have to convert the measure in degree into radians:

Maybe you should seek for the tangent-chord-theorem(?) [I don't know if this is the appropriate name in English] which states that the angle between the chord (here BC, name of angle

) is as large as the angle at the circumference (here

).