Question

What is the length of an arc of a circle that subtends 2 1/2 radians at the centre when the radius of the circle is 8cm

Answer 1

there are 2 pi radians in a circle

C = 2 pi R

so
(2.5/2pi)(2 pi R)

= 2.5 R

= 2.5 * 8 = 20

Answer 2

or, just remember that

s = rθ = 5/2 * 8 = 20