Question

in a circle of radius 21 cm and ask subtends an angle of 60° at the centre find the length of Arc

Answer 1

We have given that r = 21 cm and θ = 60⁰ and π = 3.14

Length of arc = (πrθ)/180
= (3.14 * 21 * 60)/(180)
= 3956.4/180
= 21.98 cm

Answer 2

Circle radius = R = 21 cm
Angle subtended at the center by an Arc = Ф = 60° = π/3

Length of the Arc = s = R * Ф =
 s =  21 * π/3 = 7 π
   = 2 *22/7 = 22 cm (approx)

Formula.
s = R * Ф  in radians
     or  R  * Ф*π/180°   if Ф is in degrees.