What is the formula for the circumference of a circle in terms of r?

The mathematical expression 9€ t means

Identify a1 and r for the geometric sequence an= -256(-1/4) n-1

Which is a true statement about an exterior angle of a triangle a. it is formed by two segments that are not sides of the triangle, b. it forms a linear pair with one of the interior angles of the triangle, c. it is complementary to one of the interior angles of the triangle. d. it is formed by two segments that are sides of the triangle.