Find all extreme points to$\displaystyle f\left( x,y \right)=\left( x^{2}+y^{2} \right)\left( xy+1 \right)$

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Not to important, but I'll put it here anyways. The original question is this

"Problem: 3c) Are there any other extreme points than those in 3a)? See image Skärmavbild 2012-09-13 kl. 19.02.23.png "