## Calculus (3rd Edition)

The angle between two vectors $u$ and $v$ is given by $$\cos \theta=\frac{u \cdot v}{\|u\|\|u\|}$$ Now, if $u \cdot v\lt0$, this means that $\cos \theta$ is negative, which means that the angle $\theta$ is in the second quadrant and therefore it is obtuse.