How to find the stationary point of multivariate function? For example, f (x, y) = x ^ 3-y ^ 3 + 3x ^ 2 + 3Y ^ 2-9x The answer is to find FX and FY first, and then get four stagnation points. According to the method on the Internet, I can't find the stagnation point with FX = 0 and FY = 0. I try to find the stagnation point with FXX, fxy and FYy = 0. How can I find it? Just write a clear process and method,

fx=3x^2+6x-9=3(x+3)(x-1)=0 x=-3,x=1
fy=-3y^2+6y=-3y(y-2)=0 y=0,y=2
Is my concept wrong?

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