The length of an are of a circle of radius $3 cm,$ if the angle subtended at the centre is $30°$ is $(\pi =3.14)$

Let $l$ be the length of the arc. We know that, 

Angle $\theta =\frac{l}{r},$ where $\theta$ is in radian.

Given, $r=3 cm$

$\theta ={30}^{\circ }=30\times \frac{\pi }{180}=\frac{\pi }{6}\mathrm{rad}$

On putting the values of $r$ and $\theta ,$we get 

$\frac{\pi }{6}=\frac{l}{3}\Rightarrow l=\frac{\pi }{2}=\frac{3.14}{2}=1.57\mathrm{cm}.$