Chapter 1 - Vectors - 1.2 Length and Angle: The Dot Product - Exercises 1.2 - Page 290: 25

$\theta =60^{\circ}$

We know the following: $\cos\theta=\frac{u\cdot v}{|u||v|}$ Therefore, it follows: $u\cdot v=2(1)+(-1)(-2)+1(-1)=3$ $|u|=\sqrt{((2)^2+(-1)^2+1^2)}=\sqrt{6}$ $|v|=\sqrt{((1)^2+(-2)^2+(-1)^2)}=\sqrt{6}$ $\cos\theta=\frac{u.v}{|u||v|}=\frac{3}{\sqrt{6}\sqrt{6}}=\frac{3}{6}=\frac{1}{2}$ $\theta =60^{\circ}$