Answer:

Explanation:
We know that perimeter of a circle is given by 2πr (where r is the radius of the circle).
We can say that for an complete angle of 2π the perimeter is 2πr.
Now let us consider a small section of segment making an angle β at the centre.
According to the proportionality rules the length of the small segment would be

Let the perimeter of circle be P then

Hence this is the formula for length of an arc subtending angle β.