Calculus (3rd Edition)

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

Chapter 15 - Differentiation in Several Variables - 15.4 Differentiability and Tangent Planes - Exercises - Page 790: 39

Answer

The estimate value of the maximum error in $V$ is $8.948$ ${m^3}$

Work Step by Step

The volume $V$ of a right-circular cylinder of radius $r$ and height $h$ is given by $V = \pi {r^2}h$. So, the partial derivatives with respect to r and h are $\frac{{\partial V}}{{\partial r}} = 2\pi rh$ ${\ \ }$ and ${\ \ }$ $\frac{{\partial V}}{{\partial h}} = \pi {r^2}$, respectively. Using the linear approximation, Eq. (5) the change in $V$ is estimated to be $\Delta V \approx \frac{{\partial V}}{{\partial r}}\Delta r + \frac{{\partial V}}{{\partial h}}\Delta h$ Substituting $\frac{{\partial V}}{{\partial r}}$ and $\frac{{\partial V}}{{\partial h}}$ in $\Delta V$ gives $\Delta V \approx 2\pi rh\Delta r + \pi {r^2}\Delta h$ Since $V = \pi {r^2}h$, so $2\pi rh = \frac{{2V}}{r}$ and $\pi {r^2} = \frac{V}{h}$. Therefore, $\Delta V \approx \frac{{2V}}{r}\Delta r + \frac{V}{h}\Delta h$ Hence, $\frac{{\Delta V}}{V} \approx \frac{{2\Delta r}}{r} + \frac{{\Delta h}}{h}$ If $r$ and $h$ each has a possible error of at most $5\%$, then $\frac{{\Delta r}}{r} = \frac{{\Delta h}}{h} = 0.05$ The estimate of the maximum error in $V$ is $\frac{{\Delta V}}{V} \approx 2\cdot0.05 + 0.05 = 0.15$ Thus, the estimate percentage of the maximum error in $V$ is $15\%$. If the diameter is $3.5$ m and the height $6.2$ m, then $r=1.75$ and $h=6.2$. The volume is $V = \pi {r^2}h = \pi {\left( {1.75} \right)^2}\cdot6.2 \simeq 59.651$ Thus, the estimate value of the maximum error in $V$ is $0.15\cdot59.651 = 8.948$ ${m^3}$
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