Chapter 14 - Section 14.6 - Directional Derivatives and the Gradient Vector - 14.6 Exercise - Page 957: 8

a)$\nabla f =<2x\ln y, \frac{x^2}{y}>$ b)$\nabla f_{(3, 1)}=<0, 9>$

$\nabla f =<2x\ln y, \frac{x^2}{y}>$ $\nabla f_{(3, 1)}=<0, 9>$ $D_uf_{(3, 1)}=\nabla f_{(3, 1)}\cdot u=<0, 9>\cdot<-\frac{5}{13}, \frac{12}{13}>=\frac{108}{13}$