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

Suppose f is a smooth function of x and y, and at the point (1,1), [itex]\partial[/itex]f/[itex]\partial[/itex]x = 1, [itex]\partial[/itex]f/[itex]\partial[/itex]y = -1.

Find the unit vectorusuch that D_{u}f(1,1) = 0

2. Relevant equations

D_{u}f(1,1) = [itex]\nabla[/itex]f(1,1) *u= 0

3. The attempt at a solution

My answer was merelyu= k, which is the unit vector k of course (I don't know how to put the symbol on this. Work it out, I don't see how it is wrong. I will tell you why they said that I was wrong, and I will give my argument after an answer or two.

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# Homework Help: Test question marked wrong, even after going to office hours, but I may be right?

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