Critical Points of f(x,y)= x^{3} - 6xy + y^{3}

jegues
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Homework Statement



Find and classify all critical points of,

f(x,y)= x^{3} - 6xy + y^{3}

Homework Equations





The Attempt at a Solution



See figure attached for my attempt at the solving all the critical points. I'm getting some weird numbers so I want to make sure I did this part correct before I start classifying them.

Does anyone see any problems?

Thanks again!
 

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hi jegues! :smile:

in line 6, your 1/8 should be 1/4 :redface:

(btw, it would be a lot easier if you divided everything by 3 first)
 
You can quickly check your work using wolfram alpha... and according to it, (0,0) is one but your other answers are wrong.
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
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