{\displaystyle \displaystyle l(K)=s(b)=s(b)-\underbrace {s(a)} \limits _{=0}=\int \limits _{a}^{b}s'(\tau )\,d\tau =\int \limits _{a}^{b}{\sqrt {\varphi '(\tau )^{2}+\psi '(\tau )^{2}}}\,d\tau .}