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Question Number 203859 by Mastermind last updated on 30/Jan/24

Show that the surface                                                 z = xy  has neither a maximum nor a minimum point

Showthatthesurfacez=xyhasneitheramaximumnoraminimumpoint

Commented by deleteduser1 last updated on 30/Jan/24

For x,y→∞; z→∞  For x→∞; y→−∞⇒z→−∞

Forx,y;zForx;yz

Commented by Mastermind last updated on 30/Jan/24

I need full detail, using pdf test

Ineedfulldetail,usingpdftest

Commented by deleteduser1 last updated on 30/Jan/24

(∂z/∂y)=x;(∂z/∂x)=y... Equating to 0⇒(x,y)=(0,0)  You also have to check boundary points, which  is what is up there. Partial differentials do not  give all critical points.

zy=x;zx=y...Equatingto0(x,y)=(0,0)Youalsohavetocheckboundarypoints,whichiswhatisupthere.Partialdifferentialsdonotgiveallcriticalpoints.

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