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## Multivariable Vector: Gradient @ particular speed - Find Rate

1. The problem statement, all variables and given/known data
Given: Concentration of Fluid = F(x,y,z) = 2x^2 + 4y^4 + 2*x^2*z^2 at point (-1,1,1)
----If you start to move in the direction of Grad(F) at a speed of 8, how fast is the concentration changing?

2. Relevant equations

 I guess you should calculate $$\frac{dF}{dt} = \nabla F \cdot \frac{d\mathbf{x}}{dt}$$