Given: A circle of diameter 40 cm ⇒ radius = 20cm

The length of a chord is 20 cm.

Here, we can see that, the figure forms a equilateral triangle with side length 20 cm

(both radii = 20cm and length of chord = 20cm)

∴ ∠AOB = 60° (∵ Angles in an equilateral triangle are 60° each)

⇒ 60° = 60 × = radians

That is, The Chord AB makes radians at the centre of the circle.

Now, We know that θ = (here, θ is angle subtended by arc)

∴

⇒ l =

∴ Length of the arc AB is cm.