Let $\underline{u}(t)$ and $\underline{v}(t)$ be differentiable vector functions. Then for all $\lambda\in \mathbb{R}$, $\frac{d}{dt} [\underline{u}(t)+\lambda \underline{v}(t)]=\frac{d}{dt}\underline{u}'(t)+ \lambda \frac{ d}{dt} \underline{v}(t)$.