Calculus (3rd Edition)

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

Chapter 17 - Line and Surface Integrals - 17.1 Vector Fields - Exercises - Page 919: 30

Answer

\begin{align*} \mathbf{div(F)}&= 0\\ \text{curl} \mathbf{(F)}&=0 \end{align*}

Work Step by Step

Given $$\mathbf{F}=\left\langle\frac{x}{x^{2}+y^{2}}, \frac{y}{x^{2}+y^{2}}, 0\right\rangle$$ Since \begin{align*} \mathbf{div(F)}&=\frac{\partial \mathbf{F}_x}{\partial x}+\frac{\partial \mathbf{F}_y }{\partial y}+\frac{\partial \mathbf{F}_z}{\partial z}\\ &=\frac{\partial ( \frac{x}{x^{2}+y^{2}})}{\partial x}+\frac{\partial (\frac{y}{x^{2}+y^{2}}) }{\partial y}+\frac{\partial ( 0)}{\partial z}\\ &= \frac{-x^2+y^2}{\left(x^2+y^2\right)^2}+\frac{x^2-y^2}{\left(x^2+y^2\right)^2}\\ &=0 \end{align*} and \begin{align*} \text{curl} \mathbf{(F)}&=\begin{array}{|||} i&j &k\\ \frac{\partial}{\partial x}& \frac{\partial}{\partial y}& \frac{\partial}{\partial z}\\ \frac{x}{x^{2}+y^{2}}&\frac{y}{x^{2}+y^{2}}& 0 \end{array} \\ &= \left( \frac{\partial(0)}{\partial y}- \frac{\partial(\frac{y}{x^{2}+y^{2}})}{\partial z}\right)i - \left( \frac{\partial( 0 )}{\partial x}- \frac{\partial(\frac{x}{x^{2}+y^{2}})}{\partial z}\right)j + \left( \frac{\partial(\frac{y}{x^{2}+y^{2}})}{\partial x}- \frac{\partial( \frac{x}{x^{2}+y^{2}} )}{\partial y}\right) k\\ &= \left(-\frac{2yx}{\left(x^2+y^2\right)^2}-\left(-\frac{2yx}{\left(x^2+y^2\right)^2}\right)\right) k\\ &=0 \end{align*}
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