How do you find the length of an arc in a circle?

To calculate the length of an arc in the circle you need to measure the radius (r ), and the central angle (a)

The central angle is the angle subtended at the center of a circle by two given points on the circle.

or, the central angle is the ratio of the arc to the radius.

The length of the arc = radius X central angle

                  or s= r * a

Example: in a circle whose radius is 10 cm, and the central angle is 4/5 ... find the length of the arc

    s = r * a = 10 * 4/5 = 8 cm