積の微分法則
( f → ⋅ g → ) ′ = f → ′ ⋅ g → + f → ⋅ g → ′ {\displaystyle ({\vec {f}}\cdot {\vec {g}})'={\vec {f}}'\cdot {\vec {g}}+{\vec {f}}\cdot {\vec {g}}'}
となる。