Thomas' Calculus 13th Edition

$\approx 0.00076$ or, $\approx 0.0008$
As we know that, $u=\dfrac{v}{|v|}$ Thus, $u=\dfrac{\lt 3,6,-2 \gt}{\sqrt{3^2+6^2+(-2)^2}}=\lt \dfrac{3}{7},\dfrac{6}{7},\dfrac{-2}{7} \gt$ Now $df=(\nabla f (3,4, 12) \cdot u) ds=[(\lt \dfrac{3}{169},\dfrac{4}{169},\dfrac{12}{169} \gt)(\lt \dfrac{3}{7},\dfrac{6}{7},\dfrac{-2}{7} \gt)](0.1)$ Thus, $(\dfrac{9}{183})(\dfrac{1}{10}) \approx 0.00076$ or, $\approx 0.0008$