-
$\int \vec{F}_{ext} \cdot \overrightarrow{d l}$
-
=$\int_{A_{C_1}}^{B} \vec{F}_{ext} \cdot \overrightarrow{d l}$
-
$+\int_{B_{c_2}}^{A} \vec{F}_{ext} \cdot \overrightarrow{d l}$
-
$\int_{A_{C_1}}^{B} \vec{F}_{ext} \cdot \overrightarrow{d l}$
-
=$\int_{A_{C_2}}^{B} \vec{F}_{ext} \cdot \overrightarrow{d l}$
-
=$-\int_{B_{c_2}}^{A} \vec{F}_{ext} \cdot \overrightarrow{d l}$