f(x,y)=x*x*x+y*y*y-9xy+27极值
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f(x,y)=x*x*x+y*y*y-9xy+27极值f(x,y)=x*x*x+y*y*y-9xy+27极值f(x,y)=x*x*x+y*y*y-9xy+27极值fx(x,y)=3x^2-9yfy(x
f(x,y)=x*x*x+y*y*y-9xy+27极值
f(x,y)=x*x*x+y*y*y-9xy+27极值
f(x,y)=x*x*x+y*y*y-9xy+27极值
fx(x,y)=3x^2-9y
fy(x,y)=3y^2-9x
当fx(x,y)=0,fy(x,y)=0时,解得：x=0,y=0；或x=3,y=3
fxx(x,y)=A=6x
fxy(x,y)=B=-9
fyy(x,y)=C=6y
当x=0,y=0时,AC-B^20,AC-B^2>0
所以为极小值点,代入有：f(3,3)=0
所以函数在(3,3)处有极小值0
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f(x,y)=x*x*x+y*y*y-9xy+27极值
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