(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

See first figure.

2. Relevant equations

N/A

3. The attempt at a solution

See second figure. I defined direction of the line by which the two planes intersect as,

[tex]\vec{d}[/tex]

and found that the point they are asking about is when,

[tex]t=1[/tex]

and I'm stuck here. This is my first attempt at a question like this so I applogize in advance if I'm missing something obvious.

How do I figure out what direction is the direction of increasing x?

[tex]D_{u^{+}}f(x,y,z) = f_{x}(x,y,z)a + f_{y}(x,y,z)b + f_{z}(x,y,z)c[/tex]

So,

[tex]D_{u^{+}}f(x,y,z) = ax^{2} + b2y -c2z [/tex]

At the point (-1,1,3),

[tex]D_{u^{+}}f(-1,1,3) = a + 2b -6c[/tex]

This is as far as I got.

Any suggestions?

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# Homework Help: Directional derivatives

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