Calculus 8th Edition

Published by Cengage
ISBN 10: 1285740629
ISBN 13: 978-1-28574-062-1

Chapter 16 - Vector Calculus - 16.3 The Fundamental Theorem for Line Integrals - 16.3 Exercises - Page 1134: 4

Answer

Conservative $f(x,y)=xy^2-x^2+C$

Work Step by Step

When $F(x,y)=pi+qj$ is a conservative field, then throughout the $D$, then we have $\dfrac{\partial p}{\partial y}=\dfrac{\partial q}{\partial x}$ Here, $p$ and $q$ represents the first-order partial derivatives on a domain $D$. As we are given that $F(x,y)=(y^2-2x)i+2xy)$ Then, we have $p_x=2y; p_y=2y$ Here, $\dfrac{\partial p}{\partial y} = \dfrac{\partial q}{\partial x}$ so, the $F$ is conservative. Need to find the function $f$ such that $F=\nabla f$ The partial derivatives of $f$ are: $f_x=y^2-2x..(a) \\ f_y=2xy ...(b)$ integrate $f_x$ with respect to $x$. $f(x,y)=xy^2-x^2+g(y)$ ...(c) Here, $g(y)$ is a constant of integration with respect to $x$ and with a function of $y$. Take derivative of the equation $c$ with respect to $y$. $f_y(x,y)=2xy+g'(y) ...(d)$ On comparing the equations (b) and (d) , we get $f_y(x,y)=2xy$ and $g'(y)=0$ Now, $g(y)=C$ [ Integrate with respect to $y$] Here, $C$ is a constant of integration. Hence, $f(x,y)=xy^2-x^2+C$
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