(i) Given: l = 10cm (length of arc) and r = 75cm(radius of circle)

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

∴ θ = = radians

∴The angle by which the pendulum of length 75 cm swings if it describes an arc of 21 cm is radians.

(ii) Given: l = 15cm (length of arc) and r = 75cm(radius of circle)

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

∴ θ = = radians.

∴The angle by which the pendulum of length 75 cm swings if it describes an arc of 21 cm is radians.

(iii) Given: l = 21 cm (length of arc) and r = 75 cm (radius of circle)

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

∴ θ = = radians

∴ The angle by which the pendulum of length 75 cm swings if it describes an arc of 21 cm is radians.