How do i find an arc length using chords?

A chord on a circle will make up one side of an isosceles triangle, with the other two sides having length equal to the radius  of the circle. So if you know the length of the chord, you can find the central angle subtended by the chord (the angle between the sides with length ).

If the chord has length , then the central angle can be found with some basic trig:



Once you find , you can find the arc length using the relation