## Algebra and Trigonometry 10th Edition

$-162\sqrt 2$
We know that the angle between two vectors is: $\theta =\cos^{-1} [ \dfrac{u \cdot v}{||u|| \space ||v|| }]$ Therefore, $\cos (\dfrac{3\pi}{4}) = \dfrac{u \cdot v}{(9)(36) }$ or, $u \cdot v= 324 \times (\dfrac{-\sqrt 2}{2})$ or, $u \cdot v=-162\sqrt 2$