Using differential I want to find the value of the function $g(x,y,z) = \ln \sqrt{x^2+y^2+z^2}$ if we move from (3,4,12) to a distance of 0.1 in the direction of vector $u = 3i+6j-2k$.
I first found $$|u| = \sqrt{9+36+4} = 7$$ so $$u_o = \frac 3 7 i+ \frac 6 7j - \frac 2 7k$$ and $$\nabla g(3,4,12) = \frac 6 {29}i + \frac 8{29}j+ \frac {24}{29}k$$
Then I don't know what to do. Any ideas?