Find gradient at a point and the directional derivative Multivariable calculus

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Homework Help Overview

The discussion revolves around finding the gradient of the function f(x,y) = x^3 + 2y^3 at the point P(1,1) and calculating the directional derivative at that point in the direction of the vector A = i - j. Participants are exploring the concepts of gradients and directional derivatives in the context of multivariable calculus.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the calculation of partial derivatives and the total differential of the function. There are questions about the correct formula for the directional derivative and how to apply it using the gradient and a unit vector. Some participants express uncertainty about their initial attempts and seek clarification on the steps involved.

Discussion Status

The discussion is active, with participants attempting to clarify their understanding of the gradient and directional derivative. Some guidance has been offered regarding the relationship between the total differential and the gradient, as well as the need to compute the dot product with a unit vector. Multiple interpretations of the problem are being explored, but there is no explicit consensus on the next steps.

Contextual Notes

Participants have noted confusion regarding the calculations and the definitions involved, particularly in distinguishing between the total differential and the gradient. There is an emphasis on ensuring that the directional derivative is calculated correctly, but specific details about the unit vector or further calculations have not been provided.

Nugget3
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Calculate gradient of f
f(x,y)=x^3+2y^3
at point P (1,1)
and the directional derivative at P
in the direction u of the given vector A -> i-j

I tried to attempt this but i honestly don't know where to start. I began to take the partial derivatives of f. I got f'=3x^2dx+6y^2dy, however that answer doesn't seem correct to me.
 
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Nugget3 said:
Calculate gradient of f
f(x,y)=x^3+2y^3
at point P (1,1)
and the directional derivative at P
in the direction u of the given vector A -> i-j

I tried to attempt this but i honestly don't know where to start. I began to take the partial derivatives of f. I got f'=3x^2dx+6y^2dy, however that answer doesn't seem correct to me.

What you have calculated is the total differential of f(x,y), which includes both partials ##f_x## and ##f_y##. What is the formula for the directional derivative? Show us what happens when you try to calculate it.
 
it is the dot product of the gradient at P with a unit vector in the given direction
 
LCKurtz said:
What you have calculated is the total differential of f(x,y), which includes both partials ##f_x## and ##f_y##. What is the formula for the directional derivative? Show us what happens when you try to calculate it.

hedipaldi said:
it is the dot product of the gradient at P with a unit vector in the given direction

You ignored the part I highlighted in red.
 
try to calculate what?don't you need to calculate the directional derivative?
 
hedipaldi said:
try to calculate what?don't you need to calculate the directional derivative?

Yes. You posted a question because you were apparently stuck on the problem. You have now stated that you need to take the gradient at P dotted into a unit vector. So please do it and show us what exactly you are stuck on.
 

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