## Theorem

Let $\struct {M, g}$ be a Riemannian manifold.

Let $\closedint a b$ be a closed real interval.

Let $\gamma : \closedint a b \to M$ with $t \stackrel \gamma \mapsto \map \gamma t$ be an admissible curve.

Let $c \in \R$ be a real number such that $a < c < b$.

Let $\map {L_g} {\valueat \gamma {\closedint a b} }$ be the Riemannian length of $\gamma$ from $t = a$ to $t = b$.

Then:

$\map {L_g} {\valueat \gamma {\closedint a b} } = \map {L_g} {\valueat \gamma {\closedint a c} } + \map {L_g} {\valueat \gamma {\closedint c b} }$