Finding a Directional Derivative Given Other Directional Derivatives

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SUMMARY

The discussion focuses on calculating the directional derivative \(D_vf(P)\) given the values of \(D_if(P) = 2\), \(D_jf(P) = -1\), and \(D_uf(P) = 2\sqrt{3}\) for the vector \(u = \frac{1}{\sqrt{3}} \hat{i} + \frac{1}{\sqrt{3}} \hat{j} + \frac{1}{\sqrt{3}} \hat{k}\). The user derives the equation \(2\sqrt{3} = \nabla f \cdot \frac{1}{\sqrt{3}}(\hat{i} + \hat{j} - \hat{k})\) leading to \(6 = \frac{\partial f}{\partial x} + \frac{\partial f}{\partial y} + \frac{\partial f}{\partial z}\). The user seeks assistance in determining \(\frac{\partial f}{\partial z}\) to complete the calculation of \(D_vf(P)\) where \(v = \frac{1}{\sqrt{3}}(\hat{i} + \hat{j} - \hat{k})\).

PREREQUISITES
  • Understanding of directional derivatives in multivariable calculus.
  • Familiarity with gradient notation and vector calculus.
  • Knowledge of linear combinations of vectors.
  • Ability to manipulate partial derivatives and equations.
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  • Learn how to compute gradients and their applications in optimization.
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Students and educators in calculus, particularly those focusing on multivariable functions and directional derivatives. This discussion is beneficial for anyone seeking to deepen their understanding of vector calculus concepts.

Amrator
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Homework Statement


Suppose ##D_if(P) = 2## and ##D_jf(P) = -1##. Also suppose that ##D_uf(P) = 2 \sqrt{3}## when ##u = 3^{-1/2} \hat i + 3^{-1/2} \hat j + 3^{-1/2} \hat k##. Find ##D_vf(P)## where ##v = 3^{-1/2}(\hat i + \hat j - \hat k)##.

Homework Equations

The Attempt at a Solution


$$2\sqrt{3} = ∇f ⋅ 3^{-1/2}(\hat i + \hat j - \hat k)$$
$$= 3^{-1/2}(∂f/∂x + ∂f/∂y + ∂f/∂z)$$
$$6 = (∂f/∂x + ∂f/∂y + ∂f/∂z)$$

This is where I'm stuck. I would appreciate hints.
 
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Amrator said:

Homework Statement


Suppose ##D_if(P) = 2## and ##D_jf(P) = -1##. Also suppose that ##D_uf(P) = 2 \sqrt{3}## when ##u = 3^{-1/2} \hat i + 3^{-1/2} \hat j + 3^{-1/2} \hat k##. Find ##D_vf(P)## where ##v = 3^{-1/2}(\hat i + \hat j - \hat k)##.

Homework Equations

The Attempt at a Solution


$$2\sqrt{3} = ∇f ⋅ 3^{-1/2}(\hat i + \hat j - \hat k)$$
$$= 3^{-1/2}(∂f/∂x + ∂f/∂y + ∂f/∂z)$$
$$6 = (∂f/∂x + ∂f/∂y + ∂f/∂z)$$

This is where I'm stuck. I would appreciate hints.

##v## is a linear combination of ##i, j## and ##u##.
 
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##v = 1/\sqrt{3} <1, 0, 0> + 1/\sqrt{3} <0, 1, 0> - 1/\sqrt{3} <1/\sqrt{3}, 1/\sqrt{3}, 1/\sqrt{3}>##
##f(x,y,z) = 1/\sqrt{3} x + g(y,z)##
##f(x,y,z) = 1/\sqrt{3} y + h(x,z)##

I'm not sure what ∂f/∂z would be.
 

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