Calculus 10th Edition

Published by Brooks Cole
ISBN 10: 1-28505-709-0
ISBN 13: 978-1-28505-709-5

Chapter 11 - Vectors and the Geometry of Space - 11.4 Exercises - Page 781: 12

Answer

\[\begin{align} & \mathbf{u}\times \mathbf{v}=\left\langle -2,0,-1 \right\rangle \\ & \mathbf{u}\cdot \left( \mathbf{u}\times \mathbf{v} \right)=0 \\ & \mathbf{v}\cdot \left( \mathbf{u}\times \mathbf{v} \right)=0 \\ \end{align}\]

Work Step by Step

\[\begin{align} & \text{Let the vectors be: }\mathbf{u}=\left\langle -1,1,2 \right\rangle ,\text{ }\mathbf{v}=\left\langle 0,1,0 \right\rangle \\ & \\ & \text{Find }\mathbf{u}\times \mathbf{v} \\ & \mathbf{u}\times \mathbf{v}=\left| \begin{matrix} \mathbf{i} & \mathbf{j} & \mathbf{k} \\ -1 & 1 & 2 \\ 0 & 1 & 0 \\ \end{matrix} \right| \\ & \mathbf{u}\times \mathbf{v}=\left| \begin{matrix} 1 & 2 \\ 1 & 0 \\ \end{matrix} \right|\mathbf{i}-\left| \begin{matrix} -1 & 2 \\ 0 & 0 \\ \end{matrix} \right|\mathbf{j}+\left| \begin{matrix} -1 & 1 \\ 0 & 1 \\ \end{matrix} \right|\mathbf{k} \\ & \mathbf{u}\times \mathbf{v}=-2\mathbf{i}+0\mathbf{j}-\mathbf{k} \\ & \mathbf{u}\times \mathbf{v}=\left\langle -2,0,-1 \right\rangle \\ & \\ & \text{Now, show that }\mathbf{u}\times \mathbf{v}\text{ is orthogonal to }\mathbf{u}\text{ and }\mathbf{v} \\ & \mathbf{u}\cdot \left( \mathbf{u}\times \mathbf{v} \right)=\left\langle -1,1,2 \right\rangle \cdot \left\langle -2,0,-1 \right\rangle \\ & \mathbf{u}\cdot \left( \mathbf{u}\times \mathbf{v} \right)=2+0-2 \\ & \mathbf{u}\cdot \left( \mathbf{u}\times \mathbf{v} \right)=0\to \mathbf{u}\bot \left( \mathbf{u}\times \mathbf{v} \right) \\ & \\ & \mathbf{v}\cdot \left( \mathbf{u}\times \mathbf{v} \right)=\left\langle 0,1,0 \right\rangle \cdot \left\langle -2,0,-1 \right\rangle \\ & \mathbf{v}\cdot \left( \mathbf{u}\times \mathbf{v} \right)=0\to \mathbf{u}\bot \left( \mathbf{u}\times \mathbf{v} \right) \\ \end{align}\]
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