Let alpha(s), s in $\displaystyle [0, l]$ be a closed convex plane curve positively oriented. The curve $\displaystyle beta(s) = alpha(s) - rn(s)$, whereris a positive constant andnis the normal vector, is called a parallel curve to alpha.

Show that length of beta = length of alpha + 2(pi)r

2(pi)r is the circumference of a circle. $\displaystyle l$ is the length of alpha

Can someone help me with this?