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
).