Calculus (3rd Edition)

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

Chapter 15 - Differentiation in Several Variables - 15.5 The Gradient and Directional Derivatives - Exercises - Page 801: 20

Answer

$$16$$

Work Step by Step

Given $$ g(x, y, z, w)=x+2 y+3 z+5 w, \quad \mathbf{r}(t)=\left\langle t^{2}, t^{3}, t, t-2\right\rangle, \quad t=1 $$ since $\mathbf{r}(1)=\langle 1,1,1,-1\rangle$ and $$ \begin{aligned} \nabla g &=\left\langle\frac{\partial g}{\partial x}, \frac{\partial g}{\partial y},\frac{\partial g}{\partial z},\frac{\partial g}{\partial w}\right\rangle \\ &=\langle 1,2,3,5 \rangle \\ \mathbf{r}^{\prime}(t) &=\left\langle 2t,3t^2, 1,1\right\rangle \end{aligned} $$ Then \begin{aligned} \left.\frac{d}{d t} f(\mathbf{r}(t))\right|_{t=1}&=\nabla f_{\mathbf{r}(1)} \cdot \mathbf{r}^{\prime}(1)\\ &=\langle 1,2,3,5 \rangle \cdot\langle 2,3, 1,1 \rangle \\ &= 2+6+3+5=16 \end{aligned}
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