- #1
MuIotaTau
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Homework Statement
Identify and determine the nature of the critical points of the function $$f(x,y,z) = (x^2 + 2y^2 + 1) cos z$$
Homework Equations
##\vec{x}## is a critical point ##\iff Df(\vec{x}) = 0##
##\vec{x}## is a minimum ##\iff## every determinant of upper left submatrix of Hessian is positive
##\vec{x}## is a maximum ##\iff## every odd determinant of upper left submarix of Hessian is negative and every even determinant is positive
##\vec{x}## is a saddle point otherwise
The Attempt at a Solution
The derivative of ##f## is the row matrix with elements ##2xcosz##, ##4ycosz##, ##(-x^2-2y^2-1)sinz##.
Setting each component equal to zero, I try to find the critical point, but I feel as if something is going wrong. The first critical point (or rather, set of critical points) I find is ##(0,0,2k\pi)##, where ##k## is a positive integer. But how am I supposed to use the second derivative test on something like that? I'm just a bit lost.