Calculus (3rd Edition)

Published by W. H. Freeman
ISBN 10: 1464125260
ISBN 13: 978-1-46412-526-3

Chapter 13 - Vector Geometry - 13.4 The Cross Product - Exercises - Page 678: 61

Answer

${\bf{i}} \times {\bf{X}} = {\bf{v}}$ has a solution if and only if ${\bf{v}}$ is contained in the $yz$-plane, that is, the $i$-component is zero.

Work Step by Step

Let ${\bf{X}} = \left( {x,y,z} \right)$ and ${\bf{v}} = \left( {{v_1},{v_2},{v_3}} \right)$. We have ${\bf{i}} = \left( {1,0,0} \right)$, so ${\bf{i}} \times {\bf{X}} = {\bf{v}}$ (1) ${\ \ \ }$ $\left( {1,0,0} \right) \times \left( {x,y,z} \right) = \left( {{v_1},{v_2},{v_3}} \right)$ Consider the left-hand side of equation (1): $\left( {1,0,0} \right) \times \left( {x,y,z} \right) = \left| {\begin{array}{*{20}{c}} {\bf{i}}&{\bf{j}}&{\bf{k}}\\ 1&0&0\\ x&y&z \end{array}} \right|$ $\left| {\begin{array}{*{20}{c}} {\bf{i}}&{\bf{j}}&{\bf{k}}\\ 1&0&0\\ x&y&z \end{array}} \right| = \left| {\begin{array}{*{20}{c}} 0&0\\ y&z \end{array}} \right|{\bf{i}} - \left| {\begin{array}{*{20}{c}} 1&0\\ x&z \end{array}} \right|{\bf{j}} + \left| {\begin{array}{*{20}{c}} 1&0\\ x&y \end{array}} \right|{\bf{k}}$ So, $\left( {1,0,0} \right) \times \left( {x,y,z} \right) = - z{\bf{j}} + y{\bf{k}}$ Now, the right-hand side of equation (1) is $\left( {{v_1},{v_2},{v_3}} \right) = {v_1}{\bf{i}} + {v_2}{\bf{j}} + {v_3}{\bf{k}}$. So, equation (1) becomes $ - z{\bf{j}} + y{\bf{k}} = {v_1}{\bf{i}} + {v_2}{\bf{j}} + {v_3}{\bf{k}}$. Since ${\bf{i}}$, ${\bf{j}}$ and ${\bf{k}}$ are linearly independent, the solution is ${\bf{v}} = \left( {0, - z,y} \right)$ This implies that ${\bf{v}}$ is contained in the $yz$-plane, that is, the $i$-component is zero. Conversely, if ${\bf{v}}$ is contained in the $yz$-plane. We write ${\bf{v}} = \left( {0,{v_2},{v_3}} \right) = {v_2}{\bf{j}} + {v_3}{\bf{k}}$. Then equation (1) yields $\left( {1,0,0} \right) \times \left( {x,y,z} \right) = - z{\bf{j}} + y{\bf{k}} = {v_2}{\bf{j}} + {v_3}{\bf{k}}$ So, it has solution ${\bf{v}} = \left( {0, - z,y} \right)$. Hence, ${\bf{i}} \times {\bf{X}} = {\bf{v}}$ has a solution if and only if ${\bf{v}}$ is contained in the $yz$-plane, that is, the $i$-component is zero.
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