Calculus (3rd Edition)

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

Chapter 15 - Differentiation in Several Variables - 15.3 Partial Derivatives - Exercises - Page 780: 1

Answer

$$y^2,\ \ 2xy$$

Work Step by Step

Since $$\frac{\partial}{\partial x} f(x,y)= \lim _{h \rightarrow 0} \frac{f(x+h,y) -f(x ,y)}{h} $$ Then \begin{aligned} \frac{\partial}{\partial x} x y^{2} &=\lim _{h \rightarrow 0} \frac{(x+h) y^{2}-x y^{2}}{h}\\ &=\lim _{h \rightarrow 0} \frac{x y^{2}+h y^{2}-x y^{2}}{h}\\ &=\lim _{h \rightarrow 0} \frac{h y^{2}}{h}\\ &=\lim _{h \rightarrow 0} y^{2}\\ &=y^{2} \\ \end{aligned} and $$\frac{\partial}{\partial y} f(x,y)= \lim _{k \rightarrow 0} \frac{f(x,y+k) -f(x ,y)}{k} $$ Then \begin{aligned} \frac{\partial}{\partial y} x y^{2} &=\lim _{k \rightarrow 0} \frac{x(y+k)^{2}-x y^{2}}{k}\\ &=\lim _{k \rightarrow 0} \frac{x\left(y^{2}+2 y k+k^{2}\right)-x y^{2}}{k}\\ &=\lim _{k \rightarrow 0} \frac{x y^{2}+2 x y k+x k^{2}-x y^{2}}{k} \\ &=\lim _{k \rightarrow 0} \frac{k(2 x y+x k)}{k}\\ &=\lim _{k \rightarrow 0}(2 x y+k)=2 x y+0\\ &=2 x y \end{aligned}
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