Chapter 13 - Vector Geometry - 13.3 Dot Product and the Angle Between Two Vectors - Preliminary Questions - Page 666: 2

Obtuse

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.