There are many applications for the arc length of functions. I came across it when I needed to calculate the length of a density function in an interval $[a,b]$. The formula reads

$\displaystyle s=\int\limits_{a}^{b}\sqrt{1+\big[f'(x)\big]^{2}}\,dx$

Do you grasp how this formula arises? No? Let me show you! more →