Chapter 12 - Vectors and Matrices - 12.4 The Dot Product - Exercises and Problems for Section 12.4 - Exercises and Problems - Page 520: 13

$71.57^{\circ}$

We know that: $\cos \theta=\dfrac{w \cdot v}{|w| |v|}\\ =\dfrac{(i-j) (2i+j)}{|i-j| |2i+j|}\\=\dfrac{1}{\sqrt 2 \sqrt 5}\\=\dfrac{1}{\sqrt {10}}$ Now, $\theta = \cos^{-1} \dfrac{1}{\sqrt {10}} \approx 71.57^{\circ}$