Answer:
I haven't looked at geometry in a while, but I'm pretty sure the answer is 45°
Using the vertical angle theorem, we know that the triangle with angle a is right.
Both legs of each triangle is the length of the radius. Since both legs are the same length, both triangles are isosceles. Since both legs of each triangle are the same, and both triangles are right, they are congruent triangles.
All interior angles of each triangle is 45°, making A 45°
