Question #350679

Consider the R²-R function f defined by f(x,y)=x²+2y²-x²y.


Show that f has two saddle points

1
Expert's answer
2022-06-15T16:42:17-0400
fx=2x2xyf_x=2x-2xy

fy=4yx2f_y=4y-x^2

fx=0f_x=0fy=0f_y=0

2x2xy=02x-2xy=04yx2=04y-x^2=0

If x=0,x=0, then y=0.y=0.

If y=1,y=1, then x=2x=-2 or x=2.x=2.

Point (0,0), Point(-2, 1), Point(2, 1).



fxx=22y,fyy=4,fxy=fyx=2xf_{xx}=2-2y, f_{yy}=4, f_{xy}=f_{yx}=-2x

D=22y2x2x4=88y4x2D=\begin{vmatrix} 2-2y & -2x \\ -2x & 4 \end{vmatrix}=8-8y-4x^2

Point (0,0)


D=800=8>0D=8-0-0=8>0

fxx(0,0)=2>0f_{xx}(0,0)=2>0

Then f(0,0)f(0,0) is a local minimum.


Point (-2,1)


D=8816=16<0D=8-8-16=-16<0

Then f(2,1)f(-2,1) is a saddle point.


Point (2,1)


D=8816=16<0D=8-8-16=-16<0

Then f(2,1)f(2,1) is a saddle point.


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