Linear Algebra: A Modern Introduction

$\theta =90 ^{\circ}$
We know the following: $\cos\theta=\frac{u\cdot v}{|u||v|}$ Therefore, it follows: $u\cdot v=4(1)+(3)(-1)+(-1)(1)=0$ Whenever the dot product is zero, the angle between the vectors is a right angle. Thus: $\theta =90 ^{\circ}$